I think what you're seeing is likely an artifact of the battery pack having sat around for a long time without being charged, and may go away on its own by discharging and recharging the battery pack fully (to the point where the camera shuts off) a few times.
The charge controllers inside each battery pack store some information about the state of the battery—the number of charge cycles, information about how quickly the battery takes a charge, current cell voltage, etc. This information, combined with... IIRC the resistance across the entire set of cells, is used by smart chargers to determine how fast to charge the battery pack and to determine when to stop charging.
When a pack sits for a long time, you could potentially (IIRC) get into a situation where weak cells self-discharge at a faster rate, so that the cells are unevenly charged. In that case, the battery pack as a whole could quickly hit the point where the nearest cell is essentially full, causing charging to shut down before the other cells get fully charged. Over time, the charge should migrate to other cells, and you will be able to top up the pack, or you can just charge up the pack all the way and discharge it all the way a few times, and the charge should average out quickly enough.
Another possibility is that the charge controller crashed and lobotomized itself because it ran out of power. Assuming it came back to life (i.e. assuming you aren't getting "Camera cannot communicate with battery" errors), it could take a couple of full charge-discharge cycles for it to properly calibrate itself to the current state of the pack.
Specifically, if the pack crashed from losing power, the cycle count could actually be much higher than the battery reports. Normally, batteries recalibrate their charge indication based on their estimated full-cell capacity at the end of the last few charge cycles, so they should always says "100%" when they are fully charged, even if that represents only 75% of the battery's original capacity. However, if the battery was fully drained to the point that the charge controller forgot the cycle count and calibration data, it would have to rebuild that data over the course of several charge cycles.
Either way, it is almost certainly a battery issue. I suspect that several charge/discharge cycles in a row will either entirely fix the problem or will at least fix the reporting problem so that it reports itself as being 100% full (but you might still only get three-quarters as many shots per charge).
If it doesn't start reporting itself as 100% charged within a couple of complete discharge-recharge cycles, I would suggest replacing the battery. A battery that continues to report 75% after multiple charge-discharge cycles might have something seriously wrong with it that is causing charging to terminate prematurely, such as dendritic growths, which means it should probably be recycled ASAP before it becomes a safety problem.
That said, I am not a battery engineer, and this is all from memory, so I could be off a bit. Take this with a grain of salt.