Some believe that the governing factor is time
To me the effect of instance time became very clear when I was running the unlocked chests from Charr Reinforcements. One test showed that opening them asap after load yielded 11 items in 10 runs, while opening them after waiting 1 minute in the map yielded 42 items in 10 runs. These chests may be different from enemy npcs (but they are also different from locked chests, because they can drop between 0-3 items each), but it is possible that this is a general mechanic.
The next question is, does waiting before killing actually result in a higher yield? That depends on total run time and average increase in drop chances.
Btw, are the runs you're logging during favour or not? Either would work I think, as long as all runs are the same.
