Sudden increase in iPhone battery percentage after force restart

I’m a iPhone 12pro user from last 1 month and using ios 15.3.1 on it.

Today I charged my smartphone to 94% in the morning & before using it I decided to force restart as I was continuously using it from 10 days and haven’t rebooted it.

After restarting my phone I saw the battery percentage displayed as 100% and within few seconds it automatically dropped to 99%

Why this happened even-though my battery health is still 100%