TBH by arguing for a higher score you're tacitly denigrating Manu's full potential. If this is a C+ kind of game for Manu, then it's implied that "A+" Manu is 6 levels (B-, B, B+, A-, A, A+) above tonight's performance.
If this is a B- kind of game for Manu, then a perfect or near perfect "A+" game is only 5 levels removed from tonight's performance.
Most grades are pretty consistent on this point. They're relative to expectations for individual players. A 20 point (9-17), 6 assist, 6 rebound, 0 turnover game for Lebron James, for example, is almost never graded an 'A' by ESPN's Tom Haberstroh because Lebron can easily do so much better, whereas if Manu had such a stat line it'd almost certainly be an 'A-' performance at minimum assuming his defense was sufficiently competent. The bar for an 'A' (or 'B' or 'C') is higher for Lebron.