Why do I see discrepancies in the number of unique opens in a single mailing report

Depending on the options that you use to retrieve the report, sometimes there are discrepancies in the number of unique opens in a Single Mailing Report. Selecting Other Reports > Single Mailing Report includes Forward Contacts and Original Contacts by default. Selecting Reporting > Single Mailing Report does not. You must change All Contacts to Original Contacts in Single Mailing Report to get the numbers to match.