Hi there -- Is it possible that the other contacts in your import file were already in the account? If so, they would have been added to the group as a result of the import but wouldn't be reflected in the "contacts added" column of your import statistics because they were added to the account at an earlier date. Importing using the "add only" method, as you did here, will add all the addresses on your spreadsheet to the designated group but doesn't update the associated data for addresses already in the account. If you had used "add and update" you would likely see those contacts reflected in the "contacts updated" column there on the imports page.
Let me know if you feel like this isn't the case here!