This is what I believe to be true but may stand corrected.  All kills are counted in RANKED matches no matter what, whether you quit, host quits, or you complete a full game.  These are shown on the leaderboard.  However to complete achievements kills or 'curbstomps' in your case you must finish the game for them to count towards achievements. You may have 120+ curbstomps but because you quit a match where you had 30 curbstomp kills for example, you will still need to get another 10 for the achievement - but this will not be shown on the leaderboard. The leaderboard will state you have 120

Hope this is clear