Question about ranked placement matches.

When i first decided to play ranked, i did the 10 placement matches, I won 6 and lost 4 and was put into bronze 3 from that around march 2014. And now just today finally after 148 games from doing the promotion matches for silver 5 over and over because of bad games i get silver 5, out of bronze, and a lot of time spent into that, and I don't do that bad in my opinion. So I need an answer to this part. My wife who just now finished her last placement match with 3 wins and 7 losses got put into Silver 4. Honestly confused about the system hoping someone will explain.