It's using information it already has available.
admin:admin
Single crack : In this mode, john will try to crack the password using the login/GECOS information as passwords.
Found as a result from Google Search.
The typical format for the GECOS field is a comma-delimited list with this order:
- User's full name (or application name, if the account is for a program)
- Building and room number or contact person
- Office telephone number
- Any other contact information (pager number, fax, etc.)
In most of the UNIX systems this information can be changed by normal users using the chfn command from the shell.
From Wikipedia
If the target is more advanced in their password creation, then single crack mode will likely not get a hit. If you have a dictionary of other passwords the user has used it will prove much more fruitful.