>「自動ログイン」がONのときとOFFのときとでは...
以下のApple 社技術文書があります。
Authenticating Users
Mac OS X requires user authentication prior to accessing the system. Although the loginwindow application manages the user authentication process, it does not authenticate the user itself. The loginwindow application takes the user information specified in the login screen and passes it to Directory Services for authentication. As soon as Directory Services authenticates the user, loginwindow initiates the user session.
The loginwindow application does not prompt the user for login information if the "Enable automatic log in" option is selected. Only one user at a time may enable this option from the Users pane of the Accounts System Preferences. From this same pane, the user must also enter a valid password by clicking the Set Auto Login control.