Match Performance Grade

So I just played a game with Lux in normal blind pick match (I am level mastery 5), and the game went really well like the enemy team forced surrendered a short while after 20 minutes. I ended up doing the most damage dealt as well as most kills, 13, and zero deaths, as well as 14 assists and earned the most gold. Two of my fellow teammates did pretty good too, but not better and they both got S. I ended up with an A+. PLEASE EXPLAIN why do I never get a S anymore even though I do this well. I am really trying to get to mastery 6 but this stupid, unknown code of match performance grading RIOT has is really pissing me off! Just a side note, I only went back to the base once or twice and bought only 3 items, I don't know if my items build affects my match grade.