My iPhone 11 battery health 99% why

Your battery health is fine and is undergoing normal usage. iPhone batteries, like all rechargeable batteries, are consumable components that become less effective as they age. You should change out the battery once your Battery Health reaches below 80% which will take over 2 years to reach under normal usage. Until then enjoy your iPhone.

Here is some more reading on an iPhone battery and its performance ➔ iPhone Battery and Performance - Apple Support

Axel F.

Your Battery Health is excellent. It is an inexact measurement that varies with usage. You need not worry until it reaches 80%, at which time you will need to make an Appointment at the nearest Apple Store for a battery replacement.

https://www.apple.com/batteries/maximizing-performance/