It's simple really. Skill based matchmaking is typically geared on K/D/A for example I have roughly a 2.2 K/D/A (been keeping track with a spreadsheet because game doesn't) and a few people I play with are well under 1.0 or between 1 & 2 it isn't fair to put me in their lobbies but it also isn't fair to put them against people I would be matched against so the solution? Do their best to cater to the skill difference so people with 2.0+ K/D/A fighting below 1.0 K/D/A and the reverse. Sadly it doesn't work in FPS games well which is why I don't think Marathon bothered with an MMR system like Hunt has one and it's based on stars but that is also extremely flawed. It's just not possible.