I have added a working example of the problem which I have explained.
I have two combobox with same adapter and source yet behaves differently if the combo box is inside the grid or not.
If it is inside the grid, the valueMember gets displayed on select.
If it is stand-alone combo, the displayMembers shows up which is the correct behaviour.
Can you please tell me what is wrong in the code, here is the link of the fiddle I created.
Fiddle