I believe the reason they do this is because there will be a sixth person (one person) who is forced into a match with a five stack that has a wide range (aka one or more players learning or getting carried) and that would be unfair to the one person single queueing. Sounds good in practice, but maybe the "range" needs some tuning. I know it does for duo's.
In my case, I have a friend I play exclusively with almost every game. And we generally split roles. So I'm managing lanes and assisting team fights (because noone else will) and he's focused on objective damage (because noone else will) where noone is looking.
Until now it's worked great, and we've managed a 50% to 55% win rate, but every since the update, we get a message about a wide skill range between us two, and seemingly get placed into unwinnable matches where most people on our team don't communicate and don't understand the basic mechanics of the game. Going solo makes the games way more playable, but then I have to play a team game without my teammate.
What's crazy to me is that we generally have good stats and play as a team and together try to rally our team. We think we're equally skilled, and not having any insight into how they measure "skill" which offers us no means to resolve this.