Unfair ranking

I won 6/10 promo games and the 4/10 that I lost were due to AFKers and feeders. I carried at least 1 one of those wins too. YET I WAS PUT INTO BRONZE 5??!!?!??!?!?!?!?!?!?! I started higher last season going 3/10 wins and 7/10 losses and I was put into Bronze 3. Any explanation?