I tried this out and I do not get the same results. The display of the cell in my grid when the cell does not have focus is "1,000,000.0000", which is correct. I don't know why the value of your cell is changing. Must be something else in the code doing that.
No, I can't think of any property that would cause this. Could be something in the code like a DataFilter or an event handler on the grid.