Players A and B are friends in real life and play a game together. Player A has a rating of 900 and Player B has a rating of 1100. They queue up as a team for a 2v2 match.

Players C and D and randos on the internet and solo queue for whatever. Player C has a rating of 1900 and Player D has a rating of 2100.

The matchmaking system tries to find a match for Players A and B to play against. Their Team Rating is 900+1100 = 2000, so it looks for another team rated around 2000 to play against. The matchmaking system finds a team rated 1000, and that's no good, and a team rated 3000, and that's no good either. It can't find a team rated 2000, so it goes to the fallback plan. It just tosses the team into the solo queue and lets the solo queue matchmaking system handle it.

The solo queue matchmaking system has a player with a rating of 1900, a player with a rating of 2100, and a ~~team~~ player with a rating of 2000. Perfect! It matches the ~~team~~ player with a 2000 rating against the 1900 and the 2100, which averages out to 2000.

So our opponents were queueing as individuals and were highly skilled. So their “party Elo” was genuinely 2k. Parties would be selected to take place in a match (2v2 / 3v3 / 4v4) of similar party Elos.

We would either get paired with another couple 2k players against a full team of 2k (4v4 match) or in the worst case a 2v2 against 2k players.

In the 2v2 case, because of the Elo difference the opposing side was expected to win >99.99% of the time.

Two players of similar skill would probably get creamed by a team with a large skill disparity.