im pretty sure riot is or was using some weird match making algorithm that matches teams with similar team mmr against each other.
at least it used to be this way. but the way team mmr is calculated can be pretty messed up.
- first of all, it tried to even out the mmr of your team to your current rank. (lets say plat 1 mmr is 2200. so it tried to create teams with an average of 2200 mmr from various players with higher and lower mmr)
- your mmr can rise or fall faster than your current rank. you can be plat 1 with diamond 1 mmr. this mostly happens when you go on win streaks
- the matchmaking algorithm compensated your high mmr for your rank by putting lower and lower mmr players on your team as you kept winning, to still average a 2200 team mmr to create a "plat 1" game
this created games where the more you went on win streaks, worse and worse players would be put on your team against a usually evened out enemy team. this put basically all the pressure to carry games on your shoulders, and created situations where your team got regularly destroyed in lane and you were the one supposed to win the game despite the odds.
there were limits to how far people could be apart in mmr to be matched in the same team, and this was pretty much the breaking point you had to reach to break out of this matchmaking and get equally skilled teammates and finally climb.
im not sure if they changed the match making algorithm in the meantime. but this was the state like 2 seasons ago.
