I noticed this today. I just got done playing BTB on Pardiso and I ended the game in 2nd Place with 31 Kills 17 Deaths. But apparently, I got 40 Kills and 17 Deaths (+23 KD). So what happened to my 9 extra kills? I only killed one of my teammates once by accident. Here's the in-game report saying 31 Kills. Bungie.net : Reach : Game Details Click on Carnage Report and It says 40...
This is might be it, I had a game of doubles that listed me with 38 kills and 14 deaths, which is pretty awesome. It happened actually when the host left at the end of the game though. So either may have that effect.
Ah, I'd always wondered why that happens. It's sometimes off even on the in-game stats for your last game as well (the back button post-game). I even noticed the thing on B.net saying that host switches could screw with stats but never put two and two together when I noticed a discrepancy. Based on how Reach seems to switch host like a 10 year old channel hops, I'm guessing this is the cause of all the times I've noticed numbers being off.
Someone said on Bungie.net that destroying a vehicle counts for 2 kills in the Carnage section. I have no proof though, its just what someone said.
Look at the graphs Bnet is showing for your game. All kills and medals were achieved instantly, apparently. It just means someone quit just as the game ended, rather than waiting to get back to the lobby. (Nothing to do with weird vehicle kills)