How Does Grading Work, Does Anyone Know?

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I went Aurelion this game, and ended up with a pretty nice score. The Vlad did well too, our scores were the highest out of everyone in the game.

My bf the Varus got an S- with a score of 7/5/9 and my friend the Veigar got an S- with 9/7/11.

My score was 16/5/8 and Vlad had 18/4/4

Neither me or Vlad got S's. I would assume that if those two scores from before are S- worthy, our much higher ones should be at least S worthy. Apparently not? Like, nobody knows for sure it seems how scores are graded so it is really frustrating when I see people with lower scores getting higher grades and I have no idea why. Like what else factors into it that ended up giving them a higher grade? If I knew that, it'd make me less unhappy about it..

I've been having a hard time getting S's (I'm a support main and the only times I've gotten S's on my supports is when I have 30+ assists), and I just did really well on a champ I usually struggle with. I'm pretty disappointed I did not even get an S-, in fact, I only got an A (no +). I would assume a higher score means a higher grade. I had a good amount of gold, I won my lane, I got fed, I assisted in the taking over of objectives. Like, it is extremely frustrating when people with lower scores get higher grades than you and you have no idea how the system even grades. It always seems inconsistent.

Riot has said they don't want to reveal how it works because they don't want people taking advantage, but it's started to become frustrating. The way it grades doesn't seem consistent.