How stupid is the reward system???

ARAM Sona expectations are ridiculous. That's your explanation.

Considering Sona is probably the most played champion in ARAM (Thanks to bs ARAM only accounts) with most likely the highest win rate, the average Sona game is probably a lot higher then that. In fact I have seen many Sonas go deathless. Also why does it matter you didn't get an S? You don't get any mastery tokens for getting an S in ARAM, plus you weren't even rank 5 anyways.

Enough Sonas in ARAM perform comparably better, such that this game doesn't qualify as exceptional enough to be in the top 5% or so compared to average.

"CS harder scrub" -ARAM logic

Yeah i had a game as jhin yesterday where i went 11-4-11 and got a B+, champs that are aram OP have a lot higher average #'s on aram and so with them you've got to have a very very good game to earn a chest.

i mean I had this and it was barely enough http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2502960991/235158654?tab=overview