Not sure if this is a repost for a thread. I saw it as a post by "peterjarial" and thought it was a great read. It was something I have seen many members ask along with myself wondering what the issue was. Not 100% sure if this is factual information that we can all trust but it makes sense.
Your battery gauge is lying to you (and it's not such a bad thing) - xda-developers
Again if this is a repost, I apologize in advance.