Ohm's Law explains it all: E=IR and P=IE or volts=amps*ohms and watts=amps*volts. You can solve for the current I in the first equation which is I=E/R and substitute in the second equation changing that to P=E*E/R. So if the power stays the same but the resistance changes the voltage has to change and the same if the power changes but the resistance stays the same.
Ohm's Law is something you need to know to be a safe vaper, especially if you get into mechanical mods.