Flex Silver I matched with Bronzes

What... just... happened... I just got to S1 on Flex and had a game with my team really poorly performing for Silvers... I was really surprised, they lieterally had 40 farm in 25 minutes, everyone (Top, Jg, Mid). Pretty weird right? After game I checked their ranks and... they were B1, B2 and Unranked.

What the heck? Even if I had pretty bad MMR (and I don't think I did because I was winning a lot lately and just won promo), putting Bronzes with almost Gold is pretty ridicolous. We got destroyed pretty hard, everyone in enemy team had pretty much 150+ farm so that's huge difference.

What is that? Did that ever happen to you? Why is that a thing?