19-2-7, Not even an S

Dude, you got assists! That means you don't buy into the ranked mentality of solo carrying, and thus, cannot earn an S. Next time you should go 25-0-0 and then maybe you will be deemed forgiven and worthy of an S-

Kappa.

i think it is based on champion averages so some chars like garen with often not a lot of avg deaths you get little points for rating if you didnt die vayne avg champion kills is very high and assist prob higher than yours so your kill is over avg but your assist is below avg

have to compare to avg champion stats for all league players

Well technically the ADC is suppose to be given almost all if not all the kills