Can someone please explain this?

posting on a throwaway for what should be obvious reasons.

season 5 stats post placement

and for comparison,

season 2 stats post placements

season 3 stats post placements + 1 game

season 4 stats post placements

some context: Me, a co-worker and a few of his friends did some comp, with him and his friends being 1500 or lower, and me unranked this season. I consistently placed at 3.1-3.2k from season 2 through 4. First game, went against a full team of unranked people. We got rolled hard, and lost. The ranked people lost only 3-8 CP. Next match, 5 man premade of all diamonds, with a random plat pub. Naturally, we get rolled again because of the distinct gap of skill levels of teams.

At this point, I was surprised that the enemy team didn't have any sub 2k people in either games. Rather then doing the smart thing and stopping there, we all decided to push through and mess around till we finished all of my placement matches, out of curiosity of what i would get placed at.

So, we pushed through all 10 games. Games 2-10 all had groups of 4-5 diamonds/masters, and not a single lowbie, and we lost every single game. At the end of the 10th game, which had multiple masters mind you, we finished and waited for my rank to be handed out.

I checked early by holding down tab during POTM, and saw I got placed at mid-level masters. Keep in mind that the highest I've ever placed while playing seriously was 3,247, or somewhere near that mark. And now i got placed at 3,740, almost 500 points above my highest rank gotten by actually playing. Sadly, I didn't grab a screenshot of the actual ranking, since the party leader left the game before i could.

Honestly, I'm still in disbelief about all of this.

TL;DR- I, a diamond player, did 10 placements with 3 bronze players for shits n giggles after work. We lost all 10 matches horribly, with basically every match ended with us being spawncamped. We finally finished, and I got placed at a rank I never gotten through legit comp matches. the fuck?