volt vs amp?
You can't measure mAh with a simple voltmeter. mAh is a measure of capacity, a current sustained over a length of time. It is completely independent of voltage.
Quite literally, a 700mAh battery should be able to sustain a 700mA or .7A current for one hour before it drops below a certain voltage. It can also run at higher currents: 1400mA for 1/2 hour. 2800mA for 15 minutes (1/4 hour). 4200mA for 10 minutes (1/6 hour). 7000mA for 6 minutes (1/10 hour). That's what mAh means in a nutshell.
Of course, batteries are not 100% efficient. There will be some loss, and the loss will get bigger as the current gets higher. If you're really pushing a 700mAh cell beyond its limits, you may only get 375mAh of useful energy out of it. The rest will turn into heat, that is, the battery will get very HOT!
Another situation where a 700mAh battery may only "contain" 375mAh is if it's only partially charged. Putting the battery on the charger will fix the problem.
A 700mAh battery may only contain 375mAh if it is damaged, as well. In this case, it won't be long before the battery is completely useless, and will need to be recycled.