How do they decide the final score of a match?

I have been trying to get Jhin to lvl 7 since the mastery system levels 6 and 7 were implemented, 6 so far with 2 s left.

I get s on junglers without that much effort, even Kha zix that I used him last time almost 1 year ago, s as support are easy as hell, s as mid are too, why the hell is that hard to get S as adc? even more with Jhin.

If I farm, my team dies, if I kill, doesn't matter that the enemy team pee in their pants every time they step on my traps, doesn't matter I'm so fed they don't even want to get close to me, not even the tanks, It doesn't matter, as Ill get only a+, s- these last matches.

I have carried many matches, the worst offender was 25/5/34 100 minion kill on poro king, that was a+ 13/4/10 315 minion kills equals to s-. I know my farming with Jhin isn't diamond level, I lose minions so often, but this is starting to get quite ridiculous, what do I have to do to get S with Jhin? or even S+, that the mastery sistem will require it sooner or later? the moment my team can survive without me while I farm, the enemy team surrender.

I have even seen sivir mid going 5/6 200 minions S- rank

Seriously, has someone found a pattern, something that indicates that the system doesn't randomly set rankings? Cause for me it seems that's the anwser