Read this
About Optimised Battery Charging on your iPhone – Apple Support
Also what Apple calls "Health" is the capacity of the fully charged battery compared to a notional new battery specification.
If you only use 75% of it you may as well let your battery degrade to 75% health over 4 years, because that is what you have created for yourself.
If the phone never teaches 100% charge how is it supposed to estimate that charge as % of a new specification charge? Your 100% health rating may be misleading .
Also if you don't charge the phone all night every night on wifi, when is it supposed to do its housekeeping such as icloud backup, icloud photos etc?