Champion mastery

Is there anyone who understands how this mastery system actually works? This is not the first time this has happened to me, and it makes me kinda mad since i really don't understand it. I've read somewhere that it compares your score to scores of others on that same champion but I don't think that should make such a huge difference in the marks that you get. I got A by the way, not even A+ and warwick got S... You can see the scores yourself, and here is another screen for the damage dealt: http://imgur.com/a/kXuLW Sorry if this has already been opened but I can't find a discussion which would explain it completely. And yeah I am wood VI, so you don't have to look it up :)