i have built a mob spawner with a 4 block column and 8 blocks out. inside i have put half slabs in a pattern to prevent spiders which only spawn in a 3x3x2 area. 

the pattern is as such from the walls   (o=Air x=Slab)

ooxooxoox  oo  xooxooxoo

the spaces in the middle represent the channel of water

the pattern is also mirrored 

there are no 3x3x2 areas, could something else be causing this?