This seems to be a part of any kinda of 'sets stat to X' effect such as Hubris of the Strong and transformations with Undead and/or Rabid Rabbit. Haven't tested the Undead one specifically, though. Anyways, in these cases, it becomes apparent that it sets it to the higher of either the passive buffs, or the number it's being set to. Look for the Rabid Rabbit thread in regards to Pack Leaders to see what I mean. It can essentially become a 0/0 that's only alive because of Pack Leader buffs.