Yes that happens because the point system is based on your points vs the avg.
So if your 7000 rank and all the players in the game are 4500-5500 you will loose points for anything but first i would think . Because your so far above the AVG player score you will need to come in a good placing to sustain or gain a few points.
Where as you said you came 4th and got more points than first. Could happen in a game where all / most of the players are scored higher than you but a fair amount ( 1000+ points ) and you come in 4th . and the guy that came in first is prolly around the avg for points in the game.
kind of hard to understand but yea.