Possibly when it is on and charging, the 80% "fast" charge is displayed as 100%. If it is off it may be more accurate.

I think devices do this anyway, my iPod classic has a low battery, if you plug it in for only like 30 Min. and unplug it, typically shows half or more though I don't believe it. People don't usually drain batteries all the way, but if it looks threatening they become more conservative with the use.