Placement Ranking COMPLETE Bull!

So about a month ago now I went through my placements, in 8 of the 10 matches I was paired with and against diamonds and platinum players, carried all 5 games that I won including against those high elo players, 3 of the matches I lost were due to an afk on my team, and in all 5 games I lost I still performed very well with a good KDA in every game.

I am then demoted 4 tiers, Gold 4 to Silver 2..... obviously that right off the bat seems just completely ridiculous, and none of my other friends got demoted 4 tiers and I had a much stronger performance in almost all of my games, but I eventually blew it off to some kind of algorithmic miscalculation or something and just started to work back up through Silver a 3rd time.

However, just today a friend of mine who missed the start of the placements finally finished them. He duo'd with someone from our group of friends every single game, and he ended up going 5-5 just like me, the difference being that he fed 9 out of his 10 games and the 5 that he won were cause one of us hard carried him and his bronze/low silver teammates, and we checked, he was paired with bronze in every single game so we all assumed he'd end up bronze as well cause of his awful performance and the fact that he had the same record as me and I DERANKED 4 tiers for it.

Well he ranked up a tier.... silver 1 to gold 5..... for hard feeding 9 out of 10 games.... and getting carried by one of us for every single one of his games (the one he didn't feed he just afk jungled while every lane dominated so he never really had the chance to feed).

WTF IS THIS UTTER BS RIOT!? How can you justify this completely ridiculous outcome of the placement matches!?!?!? I have to suffer through the idiocy that is silver tier ONCE AGAIN because again I am deranked through placements even though I performed well in all my matches.... last season I went 8-2 and personally carried 7 of those wins myself and then I got deranked one rank for it..... yet my friend who underperformed in 9 out of his 10 games AND had the same W/L as me gets ranked up!? Is something wrong with my profile? Am I blacklisted or something? Or is your method of placements just utter and complete SH*T!?