I feel like the placements are based on either of two things other than wins, 1.) How well you do 2.) Prior seasons. My friend and I duoed our placements going like 7-3 or something rather and he was placed higher than me, but this was my first season in ranked whereas he had played ranked in S1 and S2. He also performed better than me K/D wise because I was support and he was ADC or Mid.
I could be way off on this but that's just my opinion on it.
My brother is awful at this game. He's averaging 5 kills/7 deaths/7 assists in 30 games and he's 3 divisions above me. I really don't understand how he isn't getting demoted or how he got placed so high in the first place. I won almost all of my placement matches and then tanked after that on some really bad streaks where I was playing horribly. I'll get back up there, but until then I'll have to listen to him brag about how much better he is than me.
Ignore the terrible job on these screen shots. It took 15 seconds to make them, lol.
Here are his last 7 games: http://i.imgur.com/xxbUP2E.jpg