Battery health

Batteries naturally lose their ability to hold a charge with time and use.

In your case, the iPhone 11 was discontinued almost a year ago. if it was actually new, it's been sitting on a shelf all that time.

Once it had time to calibrate the battery usage, it adjusted itself to the actual battery health.

It's to be expected.

Probably because the iPhone 11 was not new. Apple discontinued this model about a year ago, so if the phone really is "new", it sat in warehouse for a year. This would have been very bad for the battery.