It sort of helps to think of a battery like a balloon. When you blow into the balloon and inflate it, the balloon stretches out to allow for more capacity to accomodate the pressure. But once the balloon is inflated to the point that the rubber becomes tight, the pressure inside does not increase by a great amount, even as you continue to inflate.
Voltage can be seen as much like the balloon pressure. Once you charge the battery up to its rated voltage, you start consuming its capacity. No, it doesn't physically stretch to accomodate more, but the principal is similar. And then, there is a capacity limit too that can be exceeded and damage the battery. (overcharging)
So, when your battery is low and still putting out 7.2 volts, you can think of it as the balloon that has been deflated to JUST THE POINT that the rubber walls are tight but not stretched. Release just a little bit of the air, and the pressure will drop off very quickly as the balloon deflates to ambient pressure.
"Not enough oomph anymore".
My favorite mode? Morse, of course.