Many regulated mods downscale the voltage using PWM — a pulse-width modulation method. In short that means that the mod always fires at its maximum voltage, but does that in small pulses. Inert systems like heating coils react to pulses like the voltage is lower than maximum and the resulting voltage depends on pulse length.
There are two ways of calculating the resulting voltage for PWM mods: mean and RMS (which is actually a special case of mean function). Mods which calculate their power output in RMS fire the same power as they would in flat DC signal mode, but mods which fire in mean mode always produce more power.
The results were checked against Phil Busardo's measurements and were found to be correct.