I haven't checked this thread for a few days, so I was using the older version of the bookmark and don't have the DPUR data. I did 5 10 lap Gr.4 races at Interlagos with the following results. SR was 99/99 for all 5 races, and my qualifying time was1:38.215 (I used a TT, not a Megane).
Initial DR: 28415
Race 1: DR 28417, Started 4th, Finished 8th, Assigned Door Number 8, Net DR +2
Race 2: DR 29275, Started 1st, Finished 1st, Assigned Door Number 10, Net DR +858
Race 3: DR 29131, Started 4th, Finished 8th, Assigned Door Number 8, Net DR -144
Race 4: DR 29848, Started 1st, Finished 1st, Assigned Door Number 7, Net DR +717
Race 5: DR 31819, Started 3rd, Finished, 3rd, Assigned Dorr Number 11, Net DR +1971
I assume that the overall room strength was the cause for the differences between Race 1/3 and Race 2/4. Both of these races had identical starting and finishing positions.
After Race 5, I was promoted from DR B to DR A. Also, I gained far more DR in this race than either of my wins. Overall this was a pretty weak room with lots of C/S and D/S drivers filling out the grid and only 4 relatively competitive drivers, but 1st was significantly faster than 2nd-4th. Is there a DR bonus when you are promoted to a new rank? What other factors could explain the difference in net DR?