How the hell does this even make sense (Mastery)

Captured with Lightshot
How do masteries even work, i recently stopped playing came back played a ranked game with my friend and i guess the system thought i slightly carried. I died one time right before they surrendered and suicided in and i get an S- My CS wasnt that horrendous either a 6.6/m How do you all expect me to get my tokens if this happens smh By far the most OBJECTIVE Damage in the game Most Vision score not counting Supports, even though the score was pretty shit cause i didnt bother buying oracles that game.
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