Not sure how Riot matchmakes.

So I had ~20% winrate Gold 1 player in a platinum 2 game. I understand how matchmaking seems unfair when one team completely stomps another due to a multitude of reasons (one player just got unlucky even though they are usually good/tilt/etc), but usually in this case the match is made fair in the start. However I don't understand why a ~20% gold 1 player is in a plat 2 game. If he was like ~70% w/r gold 1 player, then I understand because their mmr would be very high. However, this person's mmr is no where close to p2 elo and it shows in his performance. Please Riot look at your algorithm.