Riot has a much higher # of champions at different levels of popularity. To do it performance biased your looking at Percentile performance. unpopular champions are more punishing to play because if your not above average you suffer. Also in a game like league, some times you have to sacrifice Stats to win. If you have to hold onto CC to deal with a
and that's part your bread and butter damage because your say a
main. Stuning Kat or Yii will benefit your team a lot more then spamming Q to raise your Damage stat. Once team fights break out, some comps you can throw Q all the time and its fine. your E is going to be good CC. Other comps you NEED to save Q for the stun. This directly impacts his stats and also his ability to make impact on a win.
Its a lot easier to play a game for stats then it is to win. spam abilitys the moment its off CD, waste gold on lots of vision. CC all the things. Constantly use skills and auto regardless of if its the right play. Play to protect your KDA. Abandon teammates to die if it keeps you alive. End of the day if you make it a goal You can pad stats very easy. That's why basically every MMR system only adds or subtracts on wins or losses.
Even in OW. It only gives you points if you win. you only climb off wins.
Also in mismatched games. Stats are deceptive. The fastest way to win in chess is a fools mate. Its very easy to do if you know how. It only happens if white is bad at the game and makes literally the very worst possible moves. stats don't show skill accurately in mismatches. In MMR it assumes it is going to mismatched new accounts. Hence why theres a higher K factor.
In Riots system. MMR gains are affected by lots of hidden factors. so I cant in good faith tell you what they are because I don't know. What I do know, if they follow the trend of Elo/MMR then the system does make matches it thinks are unfair. so the MMR gains and losses are adjusted. You win more MMR on unfavored teams. Loses less. In this system you might see some one break even, gain mmr, lose mmr. Even if they all have the same # of games and all with the same 50% win ratio.
End of the day. The fact remains its way easier to play for stats then it is to win. Padding your losses to be softer because you have good stats when at the same time your literally the guy who cost your team a win is not very fair or fun for the other 4 players on your team. Meanwhile you gain the most on wins because your allies carried your stat padding ass to a win and that also feels pretty bad.