I remember reading something about this elsewhere (can't remember where).
But the upshot of it was that the phone is designed to stop the battery from charging to 100% because overcharging damages the battery.
I'm not sure what exactly you mean about "it still charges when the charger is pulled out", but if I pull the charger out of the wall socket (leaving it connected to the phone) it carries on charging for a short while because the phone is drawing residue power from the charger.
Mine also charges for a couple of minutes when I reconnect it. Maybe because the phone has used some power in the short time it has been off charge, plus the phone would recheck the charge state of the battery as soon as you connect the charger.