I've heard similar stories before... I think it's to do with your phone's battery stats being a little off. The phone resets it's battery stats each time you fully charge, so it's not something to worry about.
If you download Current Widget
and put the widget on your homescreen, it lets you know how much power is being drawn by the battery while it is charging. You will notice that, even though the battery is showing at 100%, the phone still draws power. Once the phone reaches 0mA, you know that the phone is FULLY charged