Got ranked Bronze 4 after winning 8 out of 10 placement matches

So I had just finished doing my placement matches and I was placed in Bronze 4 after winning 8 out of my 10 placement matches, previous season, I was placed in Bronze 2 even though I only won two placement matches. I played Soraka for four of those games, and my kda's were: 0/3/25, 1/1/18, 1/5/24, and 1/3/17. For the other four games I had gone 3/0/12, 7/1/13, 0/4/24 and 11/1/7. For the two games I had lost I had gone; 6/4/17 and 0/4/21. I'm not too sure if it's entirely about playing only support because my friend had played his placements as only support and won 5 out of the 10 games but was still placed in Silver 5. I am just a bit confused about all of this.