Hit's battery has come to the end of it's short life.
As the charge becomes less, each percent becomes less in real terms.
Maybe it needs a new battery. I really don't know, but that's why I answer questions in this category for wheeled vehicles. Try posting in Consumer Electronics.
It's possible that some of the layers of the lipo battery have shorted together and needs replaced. It's very easy on an iPhone 4 or 4s just buy a battery on amazon and make sure it comes with the screw drivers to take the back off and battery connector. And remember to shut the phone off normally before disconnecting battery.
It skips the last precious percents
