Grading

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2276868859/232734382?tab=stats

I was Sona. This game was graded as an A+. WTF? Someone explain please? I'd love to see the actual grading math, IE: Kill = 10pts Death = -5pts Assist= 1pt

S+/- = 90-100 pts A+/- = 80-89 pts etc.

But 7/2/81 being an A+ bothers me.