Apple 80% charging limit

See these words in this link about the 80% Charge Limit --> About Optimized Battery Charging on your iPhone - Apple Support

"With 80% Limit enabled, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates."

If your daily schedule varies it may not be able to find a pattern for optimized charging. See:

Batteries - Maximizing Performance - Apple