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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