Batteries are very interesting devices from a scientific point of view. There is a lot going on there. In a nutshell, they are chemical reactions, so they are subject to all related physical properties.
Battery levels reported by all devices ( including ours ) have the following characteristics:
-They are lower in cold weather, much lower in freezing weather (think car batteries in Winter).
-They can recharge themselves (a bit).
-Better make batteries perform better especially in difficult environments.
What can be done about it?
-Under moderate conditions, these effects are usually not a factor.
-Using better make batteries is a good idea for everyone.
-If you are using your device in extreme weather treat it was you would your car battery, replace it at the early signs of a low level.