If doing a restore to a different SQL Server (not the one backed up) e.g., restoring a staging SQL Server db to production SQL Server db or the other way the Login and dbUser GUIDs will not match. To correct that you will need to sync the user with the login by running the script:
EXEC sp_change_users_login 'update_one', '[loginname]', '[username]'
Generally the loginname and username will be the same.
Hope this helps