When an issue is moved between projects, the original issue key is left unassigned, it is never reused, and exists solely as a searchable item pointing to the new issue key.
If the issue is subsequently moved back to the original project it is then assigned a new issue key in the project.
It would make more sense if the original issue key was reassigned to the issue.

• Recently we needed to 'undo' an incorrect move, and users subsequently queried why the number had changed.