that's not strictly true. (What i am about to write may not be gospel either, but this is my take):
Amp hours are a unit of current x time. Multiplied, not divided.
It is the electric charge transferred for a unit of time. So the higher the number referring to mAh, the more charge it can hold for a given rate of charge flow. Now I'm not sure whether this refers to the amount a battery can take for a unit of time, or if it is capable of outputting higher currents (with extended batteries) for a unit of time..... however:
The voltage between 2 points is the force that would be required to drive a particular amount of electrical energy between those 2 points. So the lower the voltage, the less energy is used up when current flows.
Usually you will find voltages being the same of very similar when comparing something so specfic as one HTC desire battery vs another HTC desire battery of a different brand. Thus when a battery has a higher mAh rating, it is possibly spreading out the current flow over a longer period of time, hence time becomes a variable (longer battery life) as opposed to a different current flow rate or energy transfer rate.
Where they differ though is in their charge capacity, (the mAh stuff), as well as possibly unmentioned other stuff like resistivity of materials, leakage ratings, outright lies about the products etc etc
I personally use the Yoobao extended battery so I can actually use my smartphone instead of turning everything off basically making my smartphone into a phone! Always check reviews before buying an extended battery.