Here's the code i'm using for one of my grids: 

                     <igtbl:UltraWebGrid ID="gvReceiver" runat="server" Width="100%" DisplayLayout-GridLinesDefault="Horizontal" Height="590px" DisplayLayout-ColWidthDefault="75px">
                    <Bands>
                        <igtbl:UltraGridBand ChildBandColumn="MEMO" Key="BandInfo"  >
                            <AddNewRow View="NotSet" Visible="NotSet">
                            </AddNewRow>
                            <Columns>
                                <igtbl:UltraGridColumn AllowGroupBy="No" AllowRowFiltering="False" AllowUpdate="Yes" DataType="System.Boolean" HeaderText="Sel" Key="chkSel" Type="CheckBox" Width="2%" >
                                    <Header Caption="Sel" Title="Sel"></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="IDNO1" HeaderText="Equipment#" Key="IDNO1" Width="10%"  >
                                    <Header Caption="Equipment#" Title="Equipment#"><RowLayoutColumnInfo OriginX="15" /></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="EQMFCD" HeaderText="Make" Width="5%" >
                                    <Header Caption="Make" Title="Make" ><RowLayoutColumnInfo OriginX="15" /></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="EQMFS2" HeaderText="Serial Number" Width="10%" >
                                    <Header Caption="Serial Number" Title="Serial Number"><RowLayoutColumnInfo OriginX="8" /></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="ACTDESC" HeaderText="Activity" Key="ACTDESC" Width="15%">
                                    <Header Caption="Activity"><RowLayoutColumnInfo OriginX="6" /></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="DATE" DataType="System.DateTime" Format="MM/dd/yyyy" HeaderText="Agrmt Date" Key="DATE" Width="8%" >
                                    <Header Caption="Agrmt Date" title="Agreement Date" ><RowLayoutColumnInfo OriginX="4" /></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="DTSHR8" HeaderText="Agrmt Date" Key="DTSHR8" Hidden="true">
                                    <Header Caption="Agreement Date"><RowLayoutColumnInfo OriginX="4" /></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="MODEL" HeaderText="Model" Key="MODEL" Width="10%" >
                                    <Header Caption="Model" Title="Model"><RowLayoutColumnInfo OriginX="1" /></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="SHIPFROM" HeaderText="From Location" Key="SHIPFROM" Width="20%" >
                                    <Header Caption="From Location"><RowLayoutColumnInfo OriginX="6" /></Header>
                                </igtbl:UltraGridColumn>
                                
                                <igtbl:UltraGridColumn BaseColumnName="SHIPLOC" HeaderText="To Location" Key="SHIPLOC" Width="20%" >
                                    <Header Caption="To Location"><RowLayoutColumnInfo OriginX="6" /></Header>
                                </igtbl:UltraGridColumn>
                            </Columns>
                        </igtbl:UltraGridBand>
                    </Bands>
                    <DisplayLayout AllowSortingDefault="OnClient" HeaderClickActionDefault="SortSingle" Name="gvContracts" RowHeightDefault="20px" RowSelectorsDefault="No" SelectTypeRowDefault="Extended" StationaryMarginsOutlookGroupBy="True" Version="4.00" AutoGenerateColumns="False" ColWidthDefault="5%" FixedHeaderIndicatorDefault="NotSet" ColFootersVisibleDefault="NotSet" HeaderTitleModeDefault="OnOverflow" GridLinesDefault="Horizontal" ScrollBarView="Vertical">
                        <FrameStyle BorderColor="InactiveCaption" BorderStyle="None" Height="590px" Width="100%" Font-Bold="True" ></FrameStyle>
                        <GroupByBox><Style BackColor="ActiveBorder" BorderColor="Window"></Style></GroupByBox>
                        <GroupByRowStyleDefault BackColor="Control" BorderColor="Window"></GroupByRowStyleDefault>
                        <ActivationObject BorderStyle="solid" BorderColor="Silver" BorderWidth="2px"></ActivationObject>
                        
                            <HeaderStyleDefault       CssClass="newIFgridheader"></HeaderStyleDefault>
                            <RowStyleDefault          CssClass="newIFgridrow" ></RowStyleDefault>
                            <RowAlternateStyleDefault CssClass="newIFaltrow"></RowAlternateStyleDefault>    
                                                                    
                        <FilterOptionsDefault RowFilterMode="AllRowsInBand" AllowRowFiltering="OnServer">
                            <FilterDropDownStyle CssClass="newIFgridfilter" ></FilterDropDownStyle>
                            <FilterHighlightRowStyle CssClass="newIFgridfilterhighlight" ></FilterHighlightRowStyle>
                        </FilterOptionsDefault>

                        <Pager MinimumPagesForDisplay="2" StyleMode="ComboBox">
                            <Style BackColor="LightGray" BorderStyle="Solid" BorderWidth="1px">
                            </Style>
                        </Pager>
                    </DisplayLayout>
                </igtbl:UltraWebGrid>

 Deleting the FrameStyle -Width property worked to get rid of the overlap for me, but now the grid resizes itself when you use the vertical scrollbar.

The resize problem goes away if you resize any of the columns manually. Any ideas on the fix for the fix? 

Deleting the FrameStyle -Width property is the key, it solved it for me. Thanks canettas.

I've used a new webGrid and found the same problem, when I set the Scroll bar to always the problem disappear...

Now using your example this is what I found:

I've found some inconsistency in your example grid......

        One column don't have the width defined and if you sum all the other columns width you get 100%....

        You set some display layout propertys in the grid object and set them again in display layout.

 Maybe they are affecting the grids functionality....

I've been able to go around this problem fast by 2 ways:

1 - use the horizontal scroll bar...

2 - delete the FrameStyle -Width property and set ScrollBar="Always"

Let me know if it helps 

Thanks for your suggestion but I still have no luck... 

My example code above didn't have the scrollbar set to always but I checked my project and I have other grids that have the scrollbar set to always that still have the scrollbar overlapping the last column.  In fact all of my grids have that overlapping problem.

Here's an example of my problem for anyone having trouble visualizing the problem:

 Still looking for solution....