To be honest, I haven't payed much attention to the voltage reading. I've just been checking ohms with new coils (which seems to be accurate) and adjusting wattage until comfortable.
Try setting it on 7W, you shouldn't notice any difference from 15W, because at either of those settings it is not regulating voltage, just dumping battery.
If you would like to be around 15W, try building your coil closer to 1.1-1.2 ohm. That way your device is actually regulating voltage.