I wonder how accurate the battery health estimates really are. While I think that a person's charging habits will impact this to a large extent, I find it hard to believe that a 3 year old battery that has been charged every night (1095 charging cycles) could be at 100% health in reality.
Discover what causes Li-ion to age and what the battery user can do to prolong its life
batteryuniversity.com
Of course I guess it depends on how "empty" you let your phone get before charging and what constitutes a "cycle."
As I've stated in another thread... I think, it's about charging behavior, but not in the way most people conclude. That is, many people (based on comments, etc) don't use their iPhones until the auto power off due to low battery. Therefore, their devices are rarely rebooted -- mainly for updates/upgrades. My OCD has me doing a full cycle as much as possible, thus my iPhone is rebooted every few days or so*, and the battery nor charge percentage has ever 'jumped'. With some complaining/reporting 1% > drops in a short period or after an OS update/upgrade, (to me) it suggests iOS only does a battery health recalibration /recalculation during a boot/reboot.
Valid hypothesis?
Anyway...
@caranddriver 3 years, 1 month, 12 days later at 85%.
* I don't use my iPhone a lot lately, mostly tablet and desktop computers. As you can see, each cycle lasts plenty of time.