Understanding flashlight performance - Identical LED and battery combination

What makes it possible for two flashlights with identical LED and battery combinations to have different lumen/runtime performance?

For example, one flashlight putting out 100 lumens for 15 hours and another putting out 100 lumens for 20 hours.

Assuming they are both true lumen/runtime specifications...

Is the efficiency in the circuitry? The heat dissipation?

I could provide an example of two flashlights if needed, of course.

Wow, no replies yet?

Variances in components. LEDs can have differing Vf (forward voltage) depending on manufacturing tolerances.

It's like seeing someone test 10 of the same light and getting different results. In one of the subforums we've seen +- 400 lumens by using different batteries in the same (overdriven) light. As you go cheaper in the realm of budget lights, having wildly varying output is common as some of these lights aren't even using the same emitter.... whatever it was they had laying around when they slapped it together. Sipik 68 is pretty notable for this, there's no telling exactly what's in there.