Two possibilities:
1. Batteries age, and the wear-and-tear is even faster if you frequently subject the battery to high levels of charge differences, e.g. if you let the charge drop to 20% and then charge it back up to 100%.
2. As a consequence, batteries often lose their calibration causing erroneous battery level readings.
Two suggestions:
1. Leave it alone. As long as your device is working, there is no need to worry about that minor detail.
2. Calibrate your battery by draining it all the way to 0% and then charging your device - although I don't recommend this since battery calibration also takes a toll on battery life.