Grading System is complete bullwack

Pardon my language but i need one more S for Lvl7 mastery (I'm silver III) This Bug is really annoying, theres obviously something wrong with the grading system My last games with veigar i have carried my team hard something like 27-11 A+ (VICTORY) 11-3-10 A (VICTORY) 13-0-3 S- (VICOTRY)

S-? 13-0 AND THEY GIVE ME S- I NEED ONE S PLEASE FOR LVL 7 Please tell me why i cant get S

Thank you

PS: I have 13 winning streak with Veigar :D