Part of the protection from overcharging built into batteries now is just such a feature.
Battery hits 100% - chip in battery says "I'm full" and battery stops taking charge. Battery sips away and at about 95-96% - chip in battery says "I need a snack" so the battery nibbles at the current (trickle charge) until it tops off again. Repeat as long as plugged in.
It all depends on where in that cycle you unplug.
If the battery drops more than 5% after being unplugged from "charged," then you have a bad battery.