I looked into it, and Apple says that it may occasionally force a charge to 100% in order to help calculate a more accurate capacity estimate. Displayed capacity is obviously an estimate, where I've heard of it going up and down.
Also - the article indicates that if plugged in, it should go between 75-80% with the 80% setting while plugged in. This isn't much different than the way Macs did it previously, where it displays between 95-100% when plugged in and then attempts to top off periodically. Current iPhones/iPads/Macs do that, but they display 100% during that period, even though it may actually be between 94-100%. I've used coconutBattery, which will show the actual state of charge level.
About 80% Limit with iPhone 15 models
With iPhone 15 models, you can choose between Optimized Battery Charging, 80% Limit, and None.
When you choose 80% Limit, your iPhone will charge up to about 80 percent and then stop charging. If the battery charge level gets down to 75 percent, charging will resume until your battery charge level reaches about 80 percent again.
With 80% Limit enabled, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates.