Actually I just realized that I was misunderstanding one thing (at least).
When the game says D damage over S seconds it probably means D damage per second for S seconds (D*S total damage), not D/S damage per second for S seconds (D damage total), which was how I was originally understanding it.
If it was the latter, then a level 7 fire curse would have lower dps than a level 6 fire curse, so it is probably the former.
