Apple iPhone

I seem to be having a minor problem with the usage stats on the settings of the iPhone. If my phone needs charging, then I plug it in until it shows the plug icon that means its fully charged. When I go into the usage setting, the time doesn't get reset, rather it says "iPhone has been charged since lass full charge", and the time gets appended to. If I charge my phone for about twice as long as it takes to be fully charged, it will usually reset. Why do the settings not change when it says its fully charged?