Occurs in 1.7.2 as well, not surprising since there wasn't a bugfix listed related to this issue
Confirmed on my server as well, definitely not a hardware/network issue with 3.0ghz quad core plenty of ram and gigabit. I'm surprised this isn't the highest voted issue as of writing as this bug is a real game stopper and prevents us from enjoying the night. Does indeed only occur when zombies can't reach a valid target, both villagers and players alike. Really sad that just a handful of zombies can cause constant "Can't keep up! ..." messages but a mountain of TNT does not (at least on our server)
That would seem like a reasonable explanation, however the entries in the killEntity statistic are only living entities (zombie,cow,sheep,skeleton,etc). So the the totals of all the killEntity stats should still be equal to the mobKills stat