The resistance of the dimmer is fixed, its the wattage of the resistor that increases. As you add more bulbs in parallel the circuit resistance gets lower, but the resistance thru any one bulb remains the same.
That's true, more watts will be dissipated in the dimmer as you add more lamps. I was only wondering if they also changed the resistance of the dimmer to keep the same dimming range.
It may well be that this was considered of minor consequence and only the wattage of the dimmer was considered.
Having said that though, to some extent the two would go hand in hand anyway, higher wattage capability = heavier wire = lower resistance anyway [for roughly similar wire lengths]