My iPhone 15 battery capacity is dropped to 99% in 26 days

In addition to muguy's excellent information, I would also point out that the battery health on the phone is given in whole numbers. So, if it's really 99.5, it's going to say 99%.

Yes, it's normal.

See also:

When to charge your iPhone or iPad - Apple Community

You can expect health to decrease by 1-2 percent each month.