The serial number listed on the Hardware details page isn't always the same as the physical serial number on the drone case: The serial number reported by many drones corresponds to the flight control circuit board inside the drone and not the physical serial number printed on the label affixed to the drone. In some cases, these numbers will match.
If your numbers do not match and you want to track the physical serial number or some other unique identifier, record it in the Registration ID, Drone Serial Number, or the Notes field on the drone's details page.