As the voltage increases, the current draw decreases. In other words, a flashlight with an input voltage of, say, 4v to 9v, will draw less current at 9v than it will at 6v. So, a hypothetical flashlight running on 3 x CR123's could run longer than the same flashlight on 2 xCR123's, even though the addition of the third cell does not increase the battery capacity.
that is precisely the case and data supports this.
IOW, if a Surefire with a Malkoff will run for 60 minutes on 2 cells, it will run not for 90 minutes on 3 cells but more like 100 or 105 minutes. You get 10-15 "free" minutes. All with the same lumens or brighter.
The runtime decrease / increase is not proportional. e.g. if 2 cells means 1 hour, 1 cell is not equal to 30 minutes or 3 cells are not 90 minutes.
3 cell configuration are better than 2 and 2 are vastly better than 1. When it comes to primaries but pretty much the same holds for Li-ion, too.
When running a Malkoff on only 1 primary, I knew you would deal with reduced lumens and increased runtime but I didn't realize just bad of a decrease. The increased runitme is not worth the drastically lower lumens. Now I am only talking about under-voltaged modules not designed to run on 3V which is what one primary offers.
I've upgraded my Hound Dog to run on 3x18650 for increased efficiency.