How does the champion mastery system know which roll you are in?

Hey guys,

The other day I was playing a game which had some trading confusion so I ended up playing Nocturne top with smite. Not a big deal since everyone was friendly about it but since I had smite I built the challenging smite and a jungle item that game. At the end of the game I got an S though and it got me wondering; if champion mastery is based on the roll that you play then how does the system know which roll you play each game. Like in my game, I took smite and had a build like any jungler would but I was laning top lane. So what roll does the champion mastery system perceive me as having played that game? I assume it can't use a roll assigned in champ select (like that of draft) since you don't get that assignment in norms and often people can trade rolls.

It would be really cool to get some thoughts from other players or anyone who might have some insight into how the system knows which roll to grade you on each game.