Why did my iPhone 15 Pro Max battery drop to 91% in 9 months?

Is it normal for the battery health to drop to 91% in that time?

No, Battery Health should drop about .5% a month on average, or stated another way about 1% every two months.

In the last two weeks, it dropped from 94% to 91%. Is this normal?

No

Make an appointment at the Apple Store or take the phone to an authorized Apple service shop to let them run some checks and tests on the battery.

You can find a list of authorized service shops in your area using this link: Find Locations