battery health drops 4% within a month

iPhone 12 Pro bought Nov 2020

battery health was 93% in Oct 2021

but drops significantly to 89% Nov 2021 by the time of the warranty getting ends!

Diagnosis run with apple support said it’s normal but a month drops 4% which just doesn’t make sense

Apple iPhone battery to be designed to drains to 80% and urge user to replace once warranty ends?