Interesting problem!
It might be worthwhile to repeat the test a couple of times. If the lights have been in storage, sometimes a battery might not have full capacity right away (at least this was true for earlier battery chemistries... not sure about lithium).

Regarding discharging the battery all of the way... it's generally not good for the battery. Of course, a good light will shut off before discharging the battery far enough to hurt the battery lifetime.

Any idea how old the lights are? In addition to having a finite number of full charge-discharge cycles, many battery chemistries have a lifetime that is independent of how little they are used. The manufacturer should be able to provide some sort of number on this.

Steve in Peoria
(I think I managed to avoid answering any questions. sorry)