The grading system is pretty complicated in the way it works. Depending on which champ you use in what role basically helps the grading system chose your grade. It depends on what position you are in on what champ, and then compares how you went with everyone else who has played that champ in that role. Or at least that's how I interpret it from one of the FAQ's written by riot.
Below is the FAQ I am talking about.
What goes into the grading system? How do you calculate it?
It’s based on the champion and role that you just played in a game. So, for example, we look at Annie mid differently to Annie support. We take a series of performance metrics and compare how you do to all other players in your region in your champion / position combination. It is percentile driven - so earning a high score means you performed in the top x% of players on that champ in that position. The grade itself governs the point gains.