AirPods serial number different after replacing one?

I recently obtained a single airpod from a 3rd party, to replace a damaged AirPod (2nd gen, right side).

Prior to the swap, the AirPods serial number appearing on my iPhone matched the number on AirPods charging case.

I followed Apple instructions to reset the AirPods (with new right AirPod) and re-pair them to the phone. They are now working fine.

However I noticed the serial number for the AirPods had changed on the iPhone, and now doesn’t match the case. e.g. charging case serial starts ‘GT2’, iPhone says serial starts with ‘GRM’.

Is this normal? Or have I been subject to some shenanigans from the seller of the replacement AirPod?