The contact load wizard processing sequence works like this:

1.  Starting with row 2 in the csv file (row 1 are column/field names), it looks for an AccountID.  If it finds one, it then looks at your SJ database.  If there is an AccountID that exists (and you have checked the "Update" box on the import wizard), it decides there is no need to create a new Account record, but will instead update any of the mapped fields related to the AccountID.

2.  Once Account records have been updated (on the mapping tool, Account records are those fields in the "top half" of the map up to the contact name, website, and any custom fields you may have added - these are listed at the bottom of the mapping tool), the wizard looks to see if EITHER the Contact Last Name, First Name field OR the seperate First Name and Last Name fields have been mapped.  If so, then the wizard looks at your SJ database.  If there is an AccountID + Contact Name combination match (and you have checked the "Update" box on the import wizard), it decides there is no need to create a new Contact record, but will instead update any of the mapped fields related to the AccountID + Contact Name combination.

If you did not map the AccountID field when loading contacts, then the wizard will create a "catch all" AccountID named "Admin" and place contacts under that AccountID.