900 mAH Mi-NH rechargeable batteries.
My mate has the H7, he uses Durecell or Energizer brand purchased from the supermarket. We use our headlamps at the same time and I notice while mine is dimming, his is still quite bright.
I'll assume for a start the capacity quoted is accurate - if so, the capacity isn't radically different to alkaline cells.
Quite possibly much is down to it's the nature of the electronics in the light.
LED Lensers typically seemed to be unregulated lights (for a given setting, they would dim as the cells discharged).
With such a circuit, it can sometimes function in a way which appears rather better if using alkaline cells rather than rechargeables, if looking at the output late in battery discharge.
With alkalines, initial output can be higher due to higher initial voltage, but since alkaline cells tend to discharge smoothly, the output gradually reduces over time in a way which can be hard to notice.
With NiMH cells, the voltage is much more stable up to the point where one or more cells is essentially empty, at which point the voltage will crash and output drops much more noticeably.
The initial stability of NiMH keeps output fairly constant (and thus keeps current draw constant and high).
Because declining output in typical unregulated circuits also means lower current draw, once the alkaline cells have discharged below the typical NiMH cell voltage, there can still be reasonable output from them combined with ever-decreasing current draw, which can give alkaline cells a very long 'tail' to the discharge.
As long as the light is still usable when output has dropped somewhat, it can seem the alkaline cells are 'better', but if you had a lightmeter and kept adjusting the NiMH light output to match that of the alkaline light, you might see essentially the same runtime performance from NiMH, at least for useful light outputs.
The alkalines may just be giving imperceptible automatic dimming from the start, with a positive effect on runtime-to-unusability.
But possibly the 900mAh cells also aren't quite what they claim.
What brand are they?
What kind of charger are you using with them?