How do Riot's placement decisions work?

Ok so, I am in no means good player. I'm not saying that I deserve a better rank and everyone else sucks compared to me. But. I understand mechanics, I play well most of the time. I fuck up, yes, we all do. Last season reflected my inability to understand the game which is why after winning only two out of my ten placement games I was put into bronze 3. This season I had a much better understanding of the game, and people who play with/against me are surprised at times when I told them I was bronze three. I went into my provisionals with high hopes, though I ended my provisionals at 4 out of 10 games won. I was excited, honestly! I thought that I was going to be at the highest bronze 2 - silver 5 and I could try to climb my way out and up to gold. However, this season I was placed in Bronze V. What decides where you get placed? I had unranked people last season who went 3-7 and were put in silver 3 so why is a 4-6 bronze 5? Is it that 2 of my 10 games had an AFK? One had someone whose entire account was made solely to troll and feed, having scores like 0/25/8 1/12/1 0/14/0. What is the process that decides your rank?