https://www.flyordie.com/games/help/tanx/en/rating_tanx.html

Not the same game but it explains the rating system in a lot of ways.

The more rating you have compared to your opponent the more points you lose if they manage to win any matches against you.
Even if you win by a lot you can still lose some rating all depending on how big a gap there is between yours and your opponents rating.

Check that link I posted it honestly explains it a lot better. It's from TANX, an entirely different game but it's still good information and will give you a rough idea of how it works lol.