As a mobile developer I can tell you this. This number is calculated by the OS and developers have access to it through the Windows Phone 8 API (specifically, the
Battery class). This number can change drastically depending on the load your phone is taking on, what's running, etc. It is not a tried and true number, but only an estimation based on what your phone is doing at the time. And even then, it's only an estimation with a margin of error. I would not worry about it too much on what the number says. I would only worry if your battery is draining rapidly or if you can't last through the day with reasonable use. If that's the case, then you have some background process or app is that is causing it. But if not, I wouldn't lose sleep over it. :)