The mastery ranks after a game are not based on general game knowledge... Each game is compared to the rest of the games of all the players in your region WITH THAT CHAMPION in the same role and the stats each one had and if your game is among the BEST game stats recorded at the time, you get an S+. all games are graded based on your in game STATS alone, CS, Kills, Deaths, Assists, Turrets destroyed, neutral minions slain, Damage done, EVERYTHING. Everything is recorded as a point of refference and each day the requirements change because people are getting better. What is worth an A+ today might be worth a B- tomorrow and an S+ today might be an A+ tomorrow. That's how your grade is given, not by your general knowledge. The game doesn't know what your knowledge is. It tracks your movements, where you spend most of your time early to decide your role, how much damage you do, how many skillshots you hit as a % of your total spellcasts, how many turrets you took, at what point in the gametime, How many times you died with each death deteriorating your potential grade, how many kills you got and so on. So, if i want to get an S+ as Zed i need to play well enough AS ZED, so that my end game stats are among THE BEST of my region.
It doesn't consider rank, division or tier. It only calculates STATS. Nothing more, nothing less.
General knowledge doesn't matter, it can't see it. Get it now?