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.
Nice to get that cleared up. Thanks.
"I prefer to call armour lock "an admission of failure" " - jimmytheballs Xbox Live Gamertag - Blue Idiot