The bug

Skeleton and wither skeleton skulls don't render the hat layer when placed or worn. Although the default textures do not have anything in the hat area, the hat layer will display properly on the mobs themselves if that area is filled in a resource pack. This was fixed for Zombies in 14w03a.

How to reproduce

1. Load this texture pack in-game.
2. Summon a skeleton and wither skeleton.
3. Place a skeleton and wither skeleton skull.
   → The skulls heads aren't read like the mobs itself