Confusing rating system

Hi, fellow rioters! I am sure this question has been asked before but does anyone know how the rating system works? In the link, you can see an image showing my two last games as Nami. In one game I was 8/4/24 (KDA 8.0, kill participation 65%) and received an S+. In the next match, my score was 1/2/31 (KDA 16.0, kill participation 86%) and ended up getting an S-. I am utterly confused to why for a better game I received a lower mark. In both games, I had almost the same count of minions, so it must be something else that influenced such grade disposition. I would appreciate if someone could explain it to me. :>