Question about my placement

I am very confused because I have just finished my 10 placement matches and ended up in Bronze IV: Amumu's Lords. I need answers about exactly why I was placed in this division. Here are the facts:

• Last season (my first season of ranked) I played my 10 placements and ended up in Bronze Division I
• I then dropped down to Bronze III after making a rookie mistake of continuing to play ranked after I was losing back-to-back games
• I refrained from playing ranked for the rest of Season 5
• Today I finished my 10 placement matches for Season 6
• I won 9 of the 10 placement matches
• I played Anivia mid and Amumu jungle for the 9 wins, the one loss I played Pantheon top
• I think there were 2 or 3 games of my placements where I did not have a single death
• I think I recieved an S rating in 4 or 5 of my placement matches
• I duo'd with someone else for 3 or 4 of my placement matches
• I believe I played extremely well in almost every single placement match
• I believe I did not get carried or fed in any of the games that I played (some of the games I was the carry) Can someone look at my case and my match history (I was unable to link it since it is currently broken) and tell me what I was doing wrong and how/why I was placed in Bronze IV. It would be very much appreciated! ^-^/