I have noticed this too.
Is it because the Vf of the emitter is higher than the nominal voltage of the cell? I have seen some reports of Cree emitters measuring over 4.6Vf. Is it asking the impossible of an electric circuit to regulate up to that voltage level, from a single 3.6V cell?
Furthermore... Is it true that part of the price for a higher end light goes into the manufacturer selecting emitters at the lower spec range for Vf?
The Vf is always around 3.6 volt, but so is the battery voltage. To do a good regulations your need some electronic that can do both buck and boost regulation and that is not very common.
The cheapest solution is only to use pwm regulation, then the max. light output will fall, when the battery is discharged.
A better solution is to do only boost and use pwm to regulate brightness, this means that the light will be a bit brighter on a fully charged battery, but maintain a good brightness, until the battery is empty (NiteCore D10/EX10 does something along these lines).
And the best solution is to have both buck and boost, then your can use a wide range of input voltages and maintain a steady light output.