Champion Mastery Grades

I am a little unclear on the newly instituted grading system. I recently played a game where my KDA was 24/5/12 with 146 cs in 36 minutes and I got an A+. However, in previous games where I played well with the same champ I got S's for much lower KDA's (14/7, 13/4, wtc) AND lost. Small difference in grade, but just wondering why a 24/5 on a winning game is worse than a 14/7 losing game.

Srsly not trying to brag about KDA's or anything... just sayin.