It has to do with the way LUX is measured. LUX is not about how many photons are emitted from the LED. That would be LUMENS. LUMENS is a rating of what the LED is emitting. LUX is a rating of how brightly illuminated is a surface on the ground a specific distance from the headlight. So, LUX takes into account the optics of the headlight, and the horizontal surface (what we usually call a road) - in front of the bicycle. So consider a Sinewave headlight. It produces a round beam if projected on a vertical surface, very different from any Busch & Müller or Schmidt beam. If a particular Busch & Müller headlight's LED emitted exactly the same number of photons as the Sinewave Beacon, at different locations in front of your bike, the two headlights would be illuminating the road surface at wildly different amounts. The Sinewave puts out an even amount of light within the circle of its beam, while the Busch & Müller headlight, when aimed correctly, puts a relatively even amount of light on the road, which of course is a horizontal surface.

Now, what about the high beam in the Ladelux and IQ-XL? Since the hub's output is limited, when the second LED is switched on for the high beam, there is less power available for the primary LED, so the LUX rating, which is a measurement of the light projected on the road surface, must be reduced.