From what I remember on previous phones, the phone charges up to 100% and stops charging for a preset amount of time. It does this because it would be bad to continually charge while its at full capacity. So if you happen to unplug right as it hits 100% it will report 100% for a while then drop down. What's most likely happening is you are unplugging it after its been charged, but has actually been using the battery for a while so as soon as you unplug it will drop to what it is really at. I think once it gets down to 95% it starts recharging.
I could be completely wrong though.