The adjustment on the panel really controls a timer that feed 12 volts (yes, 12 volts) to the plug for a short time (a few milliseconds) then nothing for a while; the average value will be what the meter reads. When you adjust the pot, you just increase the time that the 12 volts is on. In short, you are feeding the plug a pulsed 12 volt signal that is on approximately 1/10 as long a time as it is off. If you have access to an oscilloscope you can see this for yourself. The meter is reading the average current of this pulsed signal.