Its the same as when 2 players kill each other at the same time. From what I've noticed, you kill a zombie and its health drops below zero. The zombie kills you as it dies, but the server has registered the zombie death first and ModDamage records its health. Then you die and the server broadcasts the message, with the health of the zombie at that time. I think thats how it works.
Try getting killed by a zombie, then having the zombie die, the /suicide (It defaults to the last death message).