Champion mastery score

I was just curious what is the criteria for your post game champion mystery score. I recently had a game with hecarim where I did the best on the team yet got the worst score. I’ve compared scores to my friend and when i play jungle with a tank (last game i was tank and went 10-1-25) yet he had a worse score as a mid laner. Not only was the score better but my damage tanked was crazy high and my damage was comparable. why is this? This happens to me very often and my champs level up very slow.