Why is the corner where the wall meets lighter rather than darker?

They are 2 static meshes with no gap in between, so I don’t get it at all.

http://i.imgur.com/PUPN5pO.jpg

Insufficient lightmap resolution seems to be the cause.

Its most likely because the bottom of the floor mesh is allowing ambient lighting to leak in. The best way to fix this is to place another mesh under to prevent light from casting through the floor.