Usually because anything above 80% just dumps in more power but doesn't really make it any brighter. Also you appear to have a misunderstanding of how the software and hardware perform brightness reduction. If you use xLights to reduce the brightness to 70% then 70% is what you get when the controller is set to 100% brightness because the controller has no idea its at 70% when it receives the commands. So if you set it to 70% in xLights and then 50% in the controller you end up with 35% brightness. ALL brightness reduction is accomplished by changing the actual RGB byte values sent to the pixel. It doesn't not reduce voltage or anything like that. If you send 255, 255, 255 that is max brightness. If you send 128, 128, 128 that is 50% brightness. So technically if you define a half white greyscale color for an effect its equivalent to a 100% white effect set to 50% brightness. If you define an effect as full white 255, 255, 255 and then set it to 50% brightness in xLights it becomes 128, 128, 128 or maybe 127's but lets not nitpik. Then you have the controller set to 50% that effect command becomes 64, 64, 64 when it hits the pixel.
3 ways to reduce brightness/power consumption in xLights:
Change the color of the effect by reducing the RGB values equally
Lower the brightness slider in the color panel
Add a dimming curve to the model