Why is the grading so stupid?

I just played a game, ARAM, and went 14/0/25. My team had 39 kills total, I had 100% kill participation. I am annoyed and curious about the grading I got. The grade was s- but I l played perfect and I am wondering why It would only be an s- instead of s+. I received the same rating as another player who went 7/2/25.