It's just a matter of battery calibration. The built-in circuitry in the battery assembly determines the battery "charge" based on it's effective output voltage since voltage is usually a function of capacity, i.e. the potential of the battery to send out electrons. Hence, when the battery is fully charged it will deliver it's maximum output voltage. Batteries remain at maximum voltage over a period of time even if it is being used until it starts to drop (slowly at first then rapidly... like how a jet of water parabolically drops from a horizontal hose). If left un-calibrated, that initial minor drop in voltage will not be interpreted as a decrease in battery charge, in effect causing the built-in battery circuitry to "tell" the phone that the battery is still at "100%". Give it at least 20 battery charge cycles and you would see the phantom 100% no longer linger that long... with normal phone usage.