my wifes uMB does this every so often, unplug and let the battery drain for a bit or restart. usually takes care of it, unless your batter has damage i wouldn't worry about it
If I remember correctly, that is normal. The charge can fluctuate between 97 and 100 percent, to keep the battery from taking damage from being plugged in all the time.
i think this happens because the battery stops charging when its full as a way to protect the battery. the computer is running on power but still occasionally uses the battery resulting in the battery being stuck just below 100%.