I expected the mobs to spawn in the air around the mobspawner and fall down to the water Ive set up for the grinder system.

What actually happened was, that some of the mobs spawned inside the walls surrounding the mobspawner.

I did some research and it looks like the mobs spawn inside blocks, regardless of the brightness level, within a five block distance of the mobspawner.

This is a case for both skeleton and zombie spawner.