Battery health

No, overtime the battery chemically changes, it wears out, it's maximum capacity decreases.

You can replace the battery but usually, even when the health is below 90%, you will still get a great maximum capacity and great battery life, but obviously that depends on the use case

In that case I imagine it may have been a miscalculation by the operating system, or perhaps the phone is trying to recalibrate it's understanding of the batteries maximum capacity which it fairly common and would explain why it would go back up, since it hasn't finished calibrating or isn't properly calibrated yet.

It takes a few charge cycles for it to full calibrate and it does it in the background, no user interaction, which is one of the great things about apple products, usually extremely easy to use and it does everything it can in the background when possible, such as the calibration, something most users never need to think about through the entire lifetime of their products.

Yeah, there shouldn't be anything to worry about, there are a good number of explanations to why that would happen, non of them bad =)

[Sorry about the delay, there are posting limits unfortunately]