Loader during the Update rewrites data in your app data fields with values from the CSV file. However, your file may contain records that are not in your database. By default Loader imports such records into your app as new records. The switch above the Mappings table is OFF.
If you do not want such records to be imported to your app during Data Update, toggle the switch above the Mappings table to the ON position with the status 'Don't import records as new when aren't found for update'.