Confirmation bias or trend: players who join In-Progress are better, on average

[Nixxe]

It's possible that since joining in-progress is not enabled by default that people are more likely to enable it when they're more experienced with the game.

It's also possible that if someone leaves a group for a reason other than getting kicked for DCing or AFKing for too long, it's likely due to performance issues and therefore "better" is a very low bar to meet.

In either case, I can't say I've really noticed that, but I also rarely need to fill spots in active runs.