Weird, sounds like a calibration issue.
The other day mine was at 59%, i plugged it in, power cycled it and as soon as it came back on it was at 100%. Stayed there for 4 hours. LOL No joke..
Then dropped suddenly to the 80s and then 70s. It corrected it self the next day. I left it charge all night and shut it off for about 4 hours while it was charging.