I Don't Understand the Algorithm

I'm still playing a short (usually five game, but sometimes one) match or two per day at Backgammon Galaxy. My rating is so low I'm too embarrassed to share it, but my play is markedly improving. I'm winning 1/3 to 1/2 of my games and matches now.

What I don't understand is how the site awards points toward ratings.

Recently, I beat a much more highly ranked player 5 points to 1. Of the two metrics that the outcome screen shows other than victory itself, I had a better "error rating" (i.e. I made fewer mistakes), and worse "luck" (presumably this refers to things like getting dice rolls which give you no choice except to leave a post exposed). The changes to ratings that resulted were that I gained nothing and my opponent lost nothing.

Also recently, I lost 5-4 to a player whose rating was very close to mine. I had (again) worse luck and about the same error rating. On that match I lost 19.x points off my rating and the player who beat me got 19 points.

One would think that a bigger victory  against a more highly rated opponent would produce more upward movement than the downward movement produced by a close defeat against an opponent with a similar rating to one's own.

Very strange.