I have a similar question to this. I'm trying to get the 10,000 kills achievement and I've got about 7300 total so far (according to leaderboards). Thing is, when are the kills actually logged as kills towards this achievement?. Are they logged on my xbox as they happen or is it through a live server somewhere?. For example say I'm in the middle of a match with 20 kills, then the 10 year old brat who's hosting decides he's had enough and switches his 360 off before the game ends. Are those 20 kills I gained already logged towards my 10,000 some where on my 360?. Reason I ask is because I've read lots of posts by people saying they got the achievement well before the leaderboards reached the recorded 10,000 mark, eg 8500, 9000, etc. I know there have been many times where a host has cut the power mid way through a match when I've had loads of kills.
I just hope every kill I've had in a rank match has been logged towards the achievement (dont care about recorded for leaderboards) because I'm really starting to get bored playing ranked.