If a picture A is selected in the grid and picture B is selected in the filmstrip and I do something - aka switch to develop mode, or choose "edit in Photoshop" - then the action only applies to picture B. In other terms, I've lost the synchronisation between grid and filmstrip.
You can't select 1 image in the film strip and a different one in the grid view. Once you make a selection in either it carries over to the other. That is unless you hold down the Ctrl key, Windows, or I guess the Command key on a Mac. Then Both images are selected.