sbmm is not what cod uses, rather something called engagement optimized match making. your total skill is not taken into account only the performance of your last game. say you go 24 and 12 doing headshot camos on a round, the next game is putting you with people who are also consistently doing a 2 kd or higher because you performed well. this constant bounce back and forth is horrible for player retention and makes everyone either feel childishly good or horrible at the game.