Originally Posted by SouthArk370Z

Assuming you have a good charger and your car's charging system is up to snuff, all you're doing is wasting time. A good charger will not over-charge the battery, so it does no harm. If your car is properly charging the battery (charging system working and putting enough miles on it), there is no need to put a DD battery on a charger. I don't think it will increase battery life (enough to matter), but I'm not sure. Ie, you're not doing any harm, but you're not doing any good.
As for why the charger charges a fully charged battery at all: the charger setpoint may be a tenth or two of a volt above the car's and it takes longer to charge the last little bit. Or it may take it a while to realize it is already at the setpoint. YMMV