Could it have lost a little charge in the time between completion and you returning to the car, or were you watching it the whole time? And what kind of charger were you using? (Home, supercharger, public L1/L2/L3, etc.)
I charge to 80% overnight and frequently see the battery at ~79% when I get into the car. It's more pronounced when I haven't driven much the day before, because that means charging completes earlier and there's more time for "vampire loss" to kick in.