I have an S3 and my buddy has an S4 and haven't observed that behavior. When the battery is fully charged the light turns green and for all intents and purposes it has stopped charging. I imagine one of two things is happening. 1. The phone is sucking juice faster than it can replace it, therefore charging. OR 2. In Standby mode (The 4th, optional step in charging Lithium Ion is Standby mode which will occasionally apply a "topping charge".
Batteries get "mullered" by:
Charging Lithium-Ion Batteries
So, if your keeping your laptop on charge 100% of the time your going to speed up the chemical process of forming a metallic lithium layer on the anode and cause the cathode to turn into an oxidizing agent - which is what "kills" the battery.