I dont understand these mastery thingiesº

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How can i have a s- or s? Try to guess wat i got?? ill give you sometime........ I got an A- {{sticker:zombie-brand-mindblown}} GG RITO {{sticker:slayer-jinx-wink}}Plz can someone tell me why did i received an A- instead of like maybe an s- or even an A+?
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