The short answer is the control circuitry steps the voltage up by drawing higher Amps... this is why you need to make sure you are using high quality, high Amp batteries in your mods. In order for your mod to step up to 75W with a single 18650, it can draw upwards of 20A+ (depending on the mod's chip-set and the charge-level of the battery). Most mods have safety built-in to prevent them from exceeding a specific Amp range, just like they won't fire coils below a specific Ohm rating.
@jasonculp did a really good write-up on how the chip-sets bump up Wattage, Amp load on batteries, etc. and can give you a much more detailed answer... but suffice it to say, the mod's chip-set does a lot more than just display the Ohms, Voltage/Wattage, and battery-level