Old time stuff had Ammeters instead of volt meters. Ammeter measures the actual current flow. Amps and volts are closely related but are two different measurements.
You can have 14 volts while it's running and not be charging the battery. (Although it's a good enough indication that the battery is getting a charge). But it's amperage that actually charges the battery. Voltage is how much pressure is behind the amperage.
So, they are both dependant on each other in order to do different things. But, at the same time, they can also be considered independent of each other.
Myself, I'd rather be monitoring my charging with an Ammeter. It tells me more.