I hear what you're saying. My phone does the same thing. I would leave my phone charged overnight and it would say 100% when I unplug it. But after a few minutes, it would drop 1-2 percent while being idle. I disabled all running services running in the background. I notice that it starts to get better like once it gets between 80-90%. Maybe I will have to get an external charger to test it out.
Edit: Just saw this thread and it makes sense:
http://forum.ppcgeeks.com/showthread.php?t=128523