Just noticed this.. some time ago I played a match where my team's average MMR was ~2k lower than the opponents. You can imagine how the game went.
The 6,7k player told us we played fairly decent considering our Lone Druid wasn't doing anything except his way of "Solo winning the game" (AKA getting picked off every time trying to splitpush).
And pretty much their low MMR player plays a less relevant hero and then the rest of their team is all higher than us.
Also if you stack with a player who is fake 50 MMR or something your enemy ends up being really below average, so easy stomps to get party MMR up.
Another thing is that people are creating 5k MMR accounts and selling em for $50 so there's right now a wave of buying retards passing by going back all the way to 2k MMR so gotta stay off ranked for a while.

Volvo finest matchmaking.