the matchmaker calculates your chance to win, if it’s exactly 50% you gain as much SR as you lose
if it’s 60% you need to win 6 out of 10 games to stay even and if it’s 40% you need to win 4 out of 10
so what that means is you get matched against lower average SR teams more often than higher. this happens for example when you group with players of lower SR than yours