Champion mastery grade unblanced???? Please explain

For many games that i do extremely well in, i always seem to get a grade less than a -S. While this happens to me, my friends who have done worse/less than me have gotten a far better grade than i did (mainly s's). And oh trust me i checked everything; towers, objectives, kills, deaths, cs, gold, assists, damage, etc. And no matter what, it would be almost impossible for me to get an s grade. It just doesn't add up. Keep in mind that this has happened MULTIPLE times. Way too often if you ask me.
I just want an explanation on why the grade system is like this and if there is gonna be a change on it. I understand that these grades are EARNED but there are some games where I'm 99.9% that my grade should have been at least an -s I attached a screenshot of a game where I went 13/2/10 as fizz (don't worry i know fizz is op) and I had the most gold, damage given and kill participation and objective control but I only got an +A Does anyone else have the same problem? (rito pls help)