Wither Skeletons don't spawn in 2.5 high area

Wither Skeletons spawn as normal, but cannot spawn in a 2.5 block high area as before. Only normal skeletons can spawn in said 2.5 high places.

1.8 Pre1 spawns: wither skeletons in an area higher than 2.5 blocks, and normal skeletons in the 2.5 tall area.
http://i.imgur.com/TyOsQdy.png