Sounds normal to me. The system will allow a little discharge before charging it back up. normal cycle for batteries.. (your phone would do the samething if plugged in overnight just maybe not display the difference in %).
Keeping a battery at 100% is probably the worst thing for it, Im sure it has some internal circuitry that allows a slight discharge to help inscrease its lifespan.
have you tried this software to help increase its overall life even further?