I've noticed the same thing whilst boosting for the 500 wins. My partner only has 4 deaths but I have 260. Apparantly as long as it says saving when the game has finished it should all count. I don't think the leaderboards update correctly. I hope they don't anyway because me and my partner are doing the same thing every time, yet he has 4 deaths and I have 260.
Gears of War Veteran
Seriously - unlocked 6/6/07
Seriously 2.0- unlocked 2/2/10
Seriously 3.0- unlocked 21/3/13
Total Online Kills - 150,000+