Depends on the type of cells in the battery, and how they were initially charged. The older Ni-Cad type cells were notorious for developing a "memory" of charge/use cycles, and would provide only about 1/3 of their duty cycle if not fully discharged before each recharge. The newer NiMh type battery doesn't have those "memory" problems, but they do require a few initial charge/discharge cycles to come up to full expectancy. The Li-Ion type are less prone to problems also, but they too require a few initial charge/discharge cycles to become fully reliable.
Almost all of the rechargeable batteries have cells rated at 1.2 volts. Common flashlight batteries and Alkaline types are rated at 2.0 volts.
It is the nature of the chemical structure that determines the voltage/current.
The battery may be older than a year, which would indicate its life expectancy is dwindling. The battery will endure only so many cycles of use.
Another reason for poor battery life is the charger/power pack itself. It will provide more voltage than the battery needs, but only slightly. That is to ensure the battery does come to its full reserve. If the cells become too warm when charging, an internal thermal fuse may open, for safety.
But regardless of how "new" the battery is, it is still possible that one cell may have gone bad. See if there is any manufacture date on the label, and if it is still under warranty.
If not, then perhaps careful separation of the case may reveal the problem. It is not overly difficult to replace one cell in a battery that is hard to find.