Battery Status Indicator Error? Okay so I have a very interesting story to tell....

Here is what happens... I charge my phone at night, wake up in the morning, pull it off the charger, about 9-10pm it drops to about 15%, which isn't bad compared to another UN-named phone I use to use, but here is where things get interesting:

It will sit at 15% for 2-3 hours without losing a single percent, so I thought why not reboot my phone, and what happened shocked me... I expected it to suddenly be at 1-5% after reboot, but it wasn't, tonight it was at 33%... yes you read it right, it went from 15% to 33% after a reboot.

This isn't really a big issue, but I just wanted to know is this normal behavior, a software bug? or is my battery possessed by some weird battery demon?

Anyways has anyone ever experienced this or know of any way to possibly make the indicator more accurate?

Any suggestion would be helpful, oh and I have tried letting it run all the way down then charging it completely back up to 100% and this seems to have no effect...

It is Android 4.1.2
If you are rooted you can try this.

In cwm recovery, advance -> wipe battery stats.
Reboot and drain the battery fully
Recharge the battery 100 % without any interruption.