I am pretty sure now that I'm seeing a pattern of gaining/losing more points when there is more highly rated competition. For example, in a race a week ago against a field full of DR S/A/B I gained 668 pts by coming 4th of 18 (door #7), having gained 1 place. In a very mixed (A to E) field last night, finishing 4th of 17 lost me 42 points (door #5, but without the bug should've been 1, 2 or 3), having fallen back 2 places. (Both races with me as mid DR A, just slightly higher last night than a week ago, 40487 vs 38509).
I only mention the door # out of interest; I don't think it directly affects the scoring. I've certainly had some occasions where I've finished below it and still gained DR. Also, I don't think positions gained/lost affects it, but I could be wrong.