Is a bug(about DatagridColumnStyle.SetColumnValueAtRow)?

Hi all:
I created a class DatagridComboboxColumnStyle that inherits from
DatagridColumnStyle.I call the method SetColumnValueAtRow in the method
Commit.
Now,i create an item of gridcolumnstyles which contains
DatagridComboboxColumnStyle(e.g. Country).I create a row for the datagrid
set the Country is 'En',and then copy the row as the second row for the
datagrid.I changed the Country 'En' to 'Un' for the second row.The Country
of two rows will be set 'USA' when the event CurrentCellChanged first
occurs.The first row of the datasource of combobox is 'USA'.
Is this a bug?

Thanks

David