So before watchOS 10, all watches would by default use "optimised charging" and charge to 80% first and then charge to 100% before the time that you typically take the watch off the charger (based on your alarm, etc). You would basically always take your watch off the charger at 100% (unless you woke up super early without setting an alarm).
In watchOS 10 there's another feature, optimised charging limit. This will only charge your watch to 80% on days where it thinks you won't use it much. This means that on some days you'll take it off the charger and it'll be at 100%, and other days it'll be at 80%. If you tap the screen while it's charging there's an option to "charge to 100% now" if you want to guarantee it's fully charged.
On the series 9 and ultra watches, there are two settings, one for each of these behaviours—so if you want you can revert back to the pre-watchOS 10 behaviour by disabling the "optimised charging limit" and keeping the "optimised charging".
On every other model (series 8 and earlier) there is one setting that controls both behaviours. So you can either have your watch decide whether to charge fully or charge to 80%, or you can have it charge to 100% immediately every time. There is no way on these watches to get back to the previous behaviour—short of waking up a little earlier and tapping "charge to 100%" every morning manually.
Hope that clears up some confusion—I just got bit by this when my watch got down to low power mode because it decided I only needed 80% battery today.
