My kills for Warzone in my service record are dif?

I have around 900 kills in Warzone but when I come here (and HaloTracker) they tell me I have around 600. The number of games played and the total games completed are the same but for some reason the kills are not. It’s the same with Arena but with Arena the gap isn’t nearly as big (kill-wise).

BTW great game, can’t wait for the classic map playlist!