I'M SO CONFUSED!?!?!?!

So I just played a ranked game, I thought I would get an S+ rank but I got an S-. I had 23/26 kill participation plus I had the most cs. I died once and I am very confused. I will link the match history if you want to check. I once heard that it is based on other players stats from other games, but I don't see many people doing this well all of the time. Normally I would just say oh well but I thought I did really well and it really confuses me why I didn't get the S+. (btw I am the vladimir) If someone could explain to me why i didn't get a S+ that would be awesome!