Riot, is there any rhyme or reason to the grades assigned for performance?

The game in the picture earned me a flat A.

I shouldn't even need a comparison to illustrate why 11/1/19 on support Bard is nothing less than S+. But for direct comparison I went 9/4/12 (a strictly worse score) the match before as the same champ in the same role with roughly the same game time and the nearly the same gold earned and got an S.

So, Riot, tell me what rules are at play so that 11/1/19 = A performance and 9/4/12 = S performance. Because any one with a rudimentary grasp of logic would infer that there freaking aren't any.

I might not have cared earlier but now that you are making me grind for blue essence and do this nonsense to achieve the higher levels of mastery you guys are responsible for a certain standard of internal logic in the system.

Link is my match history so you can see the two games. If you can find a single plausible reason why my better game got an A and the worse one got an S, please, enlighten me: http://matchhistory.na.leagueoflegends.com/en/#match-history/NA1/208075813