Why would an iPhone 4S drops from 100% charge upon unplugging it?
It’s neither a problem nor a sign of ageing.

iOS devices, since the first iPhone, have used a method of charging their Li-ion batteries, which is different from normal charging as you know it. 

Suppose your device is at 10% charge and connected to the charger. The device will charge up to 80% in 2 hours. This is called fast charge. Now the device will charge up to 100% in another 2 hours. This is called slow charge.

When the device reaches 95% actual charge, it displays “100%” as the current charge in the status bar. The device keeps charging till it reaches 100% actual charge, at which point charging is stopped and the battery discharges naturally. Note that still the displayed charge is “100%.” Now, when the battery reaches 95% while discharging, charging starts again and continues till you reach 100%.

This cycle is repeated until you disconnect the charger, at which point the displayed charge changes from “100%” to the actual charge, which may be anywhere between 95%-99%.