David Seis puts the orphans to work:
In SQL Server, a user becomes ‘orphaned’ when it exists within a database but lacks an associated login at the server level. This typically occurs when a database is either moved or restored to a different SQL Server instance. To understand why, it’s important to note that while logins are created at the server level, users are created at the database level. Each login is linked to a unique Security Identifier (SID). Therefore, during the process of moving or restoring a database, the SIDs may not align correctly, resulting in orphaned users.
Read on for a script to find and fix orphaned users.