Sometimes I don't understand what hotdogs do in the background mmr wise. Did a super nice game on Artanis. Making plays, swapping fools to start snowball teamfights, get second best damage, second siege, better damage taken than the opposing artanis, top xp on my team and same death count as everyone else: get 13 mmr while everyone else gets 40+. I should start ignoring those numbers again especially since it's qm. I just can't help it.
What you do in a game has absolutely no bearing on your MMR beyond winning or losing. Your teammates could still be in their "placement/sliding MMR" games and you are likely done with yours, or your MMR could simply just be higher, or any number of things. How well you perform has shit all to do with it.