Season 5 Prelims

Ok so I'm pretty unhappy about this. Someone explain to me how I was in bronze 3 and I won eight out of ten of my placement matches and I was put in Bronze 4,. Literally this stupidest thing I have ever seen in this game. If I had not done the placement matches I would be at least in bronze 2 and prolly in bronze 1 with eight out of ten wins. I just want to know......really would a Riot employee explain to me is 80% winning so bad I was moved down?????? #ragequitiscoming