Reasons why leveling champion masteries is frustrating

The reasons are actually pretty simple: 1. We don't how the mark is calculated. 2. Mark does not reflect actual contribution to the game. I remember having perfect score (7/0 or so), taking objectives (turrets and dragons) and still getting A or even B+. All this most likely because of low CS. As Ziggs I had score 7/3 and did destroy 5 turrents and 2 inhibs - still got A. Team mates with the same or even worse score got S-. I think it's known that for some champions it's incredibly hard to achieve a good mark where for others it's piece of cake.
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