My iPhone 15's battery health is 87% after just 303 cycles.

Let's start by reviewing what Apple says in their support document regarding batteries:

Batteries of iPhone 15 models are designed to retain 80 percent of their original capacity at 1000 complete charge cycles under ideal conditions.

iPhone battery and performance - Apple Support

Your battery is declining faster than normal because it is either defective (doubtful) or you are placing higher than normal demands on the battery (likely).

There are some real power hogs out there. What are the 5 apps that are using the most battery power on your iPhone?

Are you normally using a 5G connection?

Contact Apple Support. They are usually able to run a remote diagnostic on your iPhone and review the results with you.

Support

5G is the most power hungry setting for cellular, especially if you do not have a very strong signal. The phone spends a lot of time trying to search for a stronger connection.

What did Apple Support say when they ran the checks and tests on your phone?

We also asked.......What are the 5 apps that are using the most battery power on your iPhone?

Social apps are real power hogs as well.

Then, you are either pushing the battery really hard, or it's just bad luck, or it's a combination of both.

Do you think you can push the battery hard enough to get it below 80% in the first year?