There is an app in the store called battery health (the app creator posts here) that shows the actual battery charge vs the system reported battery charge. On my phone there is a 3.9% difference between the two figures (the actual battery charge is lower than what the phone says the level is). I think it is possible that the phone is just misreporting the available battery power. Sadly it seems like running to zero and recharging is not fixing this for some (by this i mean the % displayed).