80% is the 'ideal storage' condition for the battery. If it's always charging to 100%, the battery life is pretty dramatically shortened.
Apple enabled this new charging manager that tries to learn your patterns (if you always unplug in the AM and leave with it, it'll learn to charge to 100% before you leave, for example). You can also click the battery icon in the tray (if you have that turned on) and tell it to charge to 100% 'right now', which only takes a minute or two with a full size charger.
This is a positive thing for the life of your battery, but if you do want to disable it, there's a setting in System Settings - click the 'i' in the circle next to Battery Health, and turn off 'Optimize Battery Charging' there.
This is a positive thing for the life of your battery, but if you do want to disable it, there's a setting in System Settings - click the 'i' in the circle next to Battery Health, and turn off 'Optimize Battery Charging' there.