DolphInN The idea is that if you do better than expected, you gain elo and if you do worse than expected, you lose elo.
Why not instead of gaining elo for losing, and losing elo for winning(how you explained it), when you win a pug that's say 5-4 when you were expected to win decisively, you gain less elo than if you would've won 5-0, and you lose less elo if you were supposed to get 5-0'd but ended up getting a closer score