Hitbox of a wither skeleton is shorter than Wither skeleton itself. It's most noticable when using bow.
How to reproduce:
1) Find a Wither skeleton
2) Take a bow and aim at upper half of Wither skeleton's head (take distance into account of course)
3) Shoot -> arrow will fly through the upper half of it's head
Tested on 1.4.4 server with 1.4.5 client with enchanted bow (Power IV, Fire I, Knockback II).