Wondering why i got an a+

So, i'm playing with a 5 man team on draft pick and select singed as my jg picked ivern so we could share blue buff since singed is a god with it. In the end of the game after 44:14 we win, I was laned against a Riven and didnt get first blood but did get the first kill in top lane so that should give insight as to how top lane went :). well here's my stats 30/2/17 243 cs & 24,159 gold on charts i saw that i got 6 turrets, did 9k dmg to turrets and almost 22k dmg to objectives and my largest killing spree was 15 and finally my total dmg to champions was higher then the second highest by almost 25k. The ivern got an s- and he went 4/3/8 187 cs and 16k gold and the karma suport also got an s-... If anyone knows how i could've done better to get just an s- then please let me know.