This behavior is reproducible when the SelectionUnit property of the RadGridView is set to "FullRow".
SelectedCells collection is not of the selected row updated when new columns are added runtime. For example, you have selected the first row which has 3 cells. Then you add 2 new columns. Now the row has 5 cells but the selected cells collection still have 3.

A possible workaround which can be used is to reset the SelectedItem property of the RadGridView.

var selectedItem = testGrid.SelectedItem;
testGrid.SelectedItem = null;
this.testGrid.Columns.Add(new GridViewDataColumn() { Header = "Value3", DataMemberBinding = new Binding("Value3"), Width = 100 });
this.testGrid.Columns.Add(CreateColumn("Value4", "Value4"));
testGrid.SelectedItem = selectedItem;