Hello energizer, and welcome. The scoring system in the game bases on relativity. I don't know the core algorithm of the scoring system myself, some posts were written here and there, but mainly it is like this.
When every player joins, the game assumes that you are playing a 1v1 game with each player in the game. If you place higher than them, you gain points. If you finish below them, you lose points. The amount lost and gained for each player depends on their rating. So, if the guy who has the least rating in the game finishes first, he will get a large boost to his rating since he had beaten a lot of guys which were rating-wise better than him. Likewise, however, if the best guy in the room (highest rating) gets first place, he would get a slight boost to his rating, since it was already expected by the system that he would beat the others. The same rules apply backwards for losing points. I hope I was able to clear this up a bit.
In your example, it seems that this other player you had beaten was pretty low rated relative to the other players in the room.
Imagine how stupid the average person is. Then realize that half of the people are even stupider than that.
-George Carlin