I've noticed this too. None of my other android devices have shown this before, and the only thing I could think of was it could have been related to the wireless charging feature. I thought maybe it got a sudden (small) boost from any kind of EM. Solar flares, weird radiowaves, aliens, etc etc, lol. Radio waves can do some funny stuff from time to time.
I just checked, and this is really weird. It shows it going up while my 'screen is on'.
Maybe my Nexus 7 charges itself via solar power from the display!!! I knew this thing was bad as, but WOW!!!!