TIL total mastery points gained in a match is highly affected by how well my team does.

When you hover on the mastery circle, it shows how many points you contributed to everyone's Champion Points this game.. I think.

Everyone in the team gets the same amount of points, some players contribute more to the total score by getting high grades like A+.

Your mastery score is more dependent on winning and how your team does than it is on you and your personal grade.