mAh means milliampere/hour: 2000mAh battery means that your battery can give a current of 2000mA for 1 hour, than the battery is empty. It is a measure of capacity, this has nothing to do with the voltage, a measure of potential drop.
An ideal battery keeps the voltage between poles constant. Real life batteries lowers their voltage while discharging, and camera hardware constantly measures the battery voltages because it's known that at a certain voltage level the battery is over and it's better to turn off the camera before that point to avoid problems. CHDK just uses the full charge tension and a nearly discharged tension to show you the level of charge remaining in your camera.
I mean you should just leave voltage levels at default if you don't see strange behaviors like CHDK indicating a non full battery level with fully charged batteries, or you camera turns off before CHDK indicates empty batteries.
sorry for my poor english.