I think it is bullets coming apart due to high velocity for that particular bullet.
I also think that high velocity with any kind of expansion can make a mess.
A bullet staying in the animal (no exit wound) thereby expending all of it's energy in the animal will make a bigger mess than one that penetrates clear through, taking some of it's energy with it.

Democracy is not freedom. Democracy is two wolves and a lamb voting on what to eat for lunch. Freedom comes from the recognition of certain rights which may not be taken, not even by a 99% vote.
*Marvin Simkin* L.A. Times (1992)