What I mean is, at the end of the game, the scoreboard might show you with 19 kills. When you get back to the menus, and bring up the post game stats, you see 22 for your total kills. This happens well over 50% of the time, and I can't imagine what causes it. I know there are rare occurences where the game has to give someone credit for a kill even if it wasn't actually a player's attack that killed someone, but those are rare. Weird technicalities like that don't happen every game, let alone multiple times in a game.
Something even weirder happened to me, for the first time. I got the opposite, where my score at the end of the game was higher than what the post game stats said. My score in game showed 17 or something, but when I looked at the stats, it listed 13 kills.
As people have already said in this thread, destroying an enemy occupied vehicle counts as an extra kill in the post game stats, NOT the actual game stats.
Destroy a warthog with 3 passengers= 4 kills, destroy a warthog with just a driver= 2 kills. So total number of enemies killed + total number of enemy occupied vehicles destroyed= final kill score. Also, say you don't die at all in a slayer game, get 14 kills with one of those being an enemy that was in a vehicle that you blew up, your final score would be 15 kills, and you will actually get +1 on your perfection commendation for that.