How is this not a S?

Dear Riot and other readers, i just finished a game with thresh, lucian and i comepletely destroyed the enemy botlane (bard and kog) and both of us had awesome scores (me: 3/0/16, Lucian 12/0/9). Lucian got an S-, which seems normal to me, but i got an A+...... Now i am asking, what do i have to do to get that loved S???? How hard do we have to destroy the enemy?? do i have to have 20 kills? i feel like the system is a bit broken. if someone could explain then yes plz. greetings, an annoyed thresh main
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