iPhone battery health is 97%. Why??

Your Battery Health is excellent. Battery Health is an inexact figure that varies with usage. All batteries degrade with time and 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 Battery Replacement. Check this link for tips...

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

Unless you and your friend are using your phone at the exact same time, the exact same places and in the exact same way with the exact same apps on the phone, comparing the two is meaningless.

Your phone is fine.