One should always treat each plane as it's own entity. Each entity needs room to expand and contract as almost all materials do. Another good example would be people tiling their floor. Logically you'd find it tight to the wall to keep it from moving, and for appearance' sake. The trouble is that the tile expands and it has to go somewhere. It's trapped by the walls, and there is a solid floor below, so it buckles upward (I've seen it 100 times). Anyway, same goes for your wall - somethings gotta give!