Why On Earth is This Not an S???

I just finished a game where I was so satisfied with my performance that I was stoked to see my post game score. While I know that K/D/A isn't everything in your rating, I find that with my overall contribution an A+ rating doesn't cut it. With a 12/2/9 KDA I killed 3 Dragons and 1 Baron and took 3 Towers. A+...