Have you tried the "Help" icon in the taskbar above and then the FAQ, lots of info there on many subjects, including ratings. FAQ
Basicly RHP expects you to beat people lower rated than you are and to lose (most of the time!) to people higher rated than you. If you lose/draw a game to a lower rated player your rating will probably go down. If you draw/win a game with a higher rated player your rating will go up. The difference in your ratings (1200 v 1400, say) is also a factor. Knock off someone 500 points above you and you get a lot of rating points, but not so much if your opponent is only 10 points above you.
(edit) Man, I have got to learn to type faster! 😠