So I just got done with a normal match. Snowballed hard. And it gives me a freaking A?! I sware this rating system is stupid.
The rating system doesn't go based of KDA or gold or anything like that.
That's a common misconception.
I'm not 100% sure about this, but it seems to goes based off your proficiency with your champion using different measuring tactics like how many skillshots you used and actually connected, your mana control, average life to damage taken ratio; stuff like that.
For example, if I play Zyra, even if I go 6/10/4 and lose, I still get either an A or a S ranking unless I literally do nothing but sit in lane and soak XP. Likewise, if I play Volibear or Ashe, champions I'm really not that comfortable with, I tend to barely get a B or C despite have a positive KDA and the most damage or gold on my team.
I think Riot did this to stress how important it is, not just to win the game, but to also understand your champion and how to play them in general.
(i was also really surprised to see you win all those game with those builds, but then i looked up the ranking. lmfao)