But precisely what you are adjusting is different. Variable wattage vs. variable voltage are different electrically...
And you can't keep resistance constant. There is variability across equipment even when they are rated the same, and resistance of a carto often changes as it is used. So as you go, with variable voltage the voltage will not change unless you change the setting, but wattage certainly can and will, and with variable wattage it is the opposite....
Let me put it this way, the kick varies the voltage automatically to keep a constant wattage. If resistance changes, the kick changes to voltage applied so that the wattage keeps constant.
The whole point of a kick is to provide the same wattage to different loads, so calling it variable wattage does not describe what it does.