I failed to explain this properly, but the car does not actively maintain the exact charge the whole time while plugged in. It
internally connects and disconnects from the charger to "top up" after it falls back down a bit, but otherwise doesn't work super hard to maintain exactly the percentage you set. This is especially confusing because a lot of historical posts on this forum falsely state that it does keep connected to wall power whenever plugged in. For what it's worth, this topping-up is how
most Lithium Ion battery chargers work - they disconnect after charging, but will reconnect to top up after the charge falls by a certain amount. Being 3% off before topping up is a bit more than I'd expect, but I've seen 4% once. Truthfully I haven't concretely figured out when they decide to top it back up, other than knowing this is how it behaves
As for why it's always been 80/81% before, my guess is that it may have done one of these "top-ups" just before you got to the car, but
this time it hadn't gotten around to that yet, so you saw it at the lower state.
Apologies for the wordiness. Succinct, I am not.
Nice
if you're ever out this direction again I can always recommend things. But bring or borrow a CHAdeMO adapter, it's hard to get around the most beautiful parts on just the Supercharger network!