Rating System Problems

I dont understand how I only got an A+ for this game.... I didnt get carried I missed a lot of cs early on and my cs is like 130 under where it probably SHOULD be if I was farming almost perfectly, but still the rating system should NOT be based SOLELY on CS and Deaths... and thats what it feels like. I got an S on my last Caitlyn game when I had 91 cs at 16 mins, which if you scale that up linearly to 26 mins that comes out to right around 174 actually maybe even a little less, I also Died ONE time in that game. I feel like if you have 0 deaths the game assumes you have a 0 kda because you cant divide by 0 and the rating system cant understand that, thats the only explanation I can think of.