the % chance the matchmaker calculates is what determines your SR gains and losses
for example if it was exactly 50% each game you would have ended right where you started
but if you chance to win was 60% in each game you’d have to win 6 out of 10 games to get even
for 40% it’s 4 out of 10 and so on
the win chance is calculated by the SR differences of both teams and probably some voodoo involving MMR