To that I can actually answer: consider it as 2 parameters: roster value and player value. Player value is how well do you plan, how well do you execute, and how often you actually play (someone who plays all the matches will have higher score than someone who only play at 5v5 etc.)
The total skill rating is the sum of all those parameter. That is, your opponents might have better roster, but you are a better player: you plan better, you execute better, you care more and therefore you achieve more, equivalent to 2M GP more...