Wondering about the data.

My guess is that it's used to keep the game synched. The server uses that number of hp, and when it runs out, you get your kill. But to make things easier, and try to have less lag, your pc already knows if the monster's going to die. You send the server the damage, and at the end: "Monster died"? If it says yes, exp gotten. If not, something fishy's going on... Also, this could help a LOT for hp bars on monsters. You don't need a server sided info, except the initial hp the monster had if it was attacked before you. Your pc can easily do the math.

I repeat again: this is just my guess.