Hostile mobs moving slow when you're at a higher altitude

(Note, this isn't the same as MC-133.)
When I am at a higher altitude than mobs chasing me (in this case I'm on a 2-high cobblestone wall), they will move very slowly. Once I step off the wall, they will move very quickly to compensate for it. It's much like a very laggy SMP server. The 2nd screenshot was taken about a 1/2 second after falling. So far, I've only observed this to happen on superflat.