S Rank....

exactly

meanwhile i have tokens and no blue gems to pay with grrr.

{quoted}

Sort of, Its graded against the current average of all the other player's stats with that champion. And since 6&7 mastery came out, everyone is on their main champion going all try-hard to get the mastery. So the current averages have gone up :P

So they are data mining what everyone mains, and getting a winrate for those champions :|. Actually rather ingenious way to get that data.