When you view a single mailing report, you might notice that the Unique Total that is being reported for Opens and Clicks is 1, but the Gross Total is higher. For example, in the hundreds or thousands, depending on the size of the list and the open or click rate.
On the Mailing Report screen, you might see the label, Tracking Level: Aggregate. This label means that when the email was sent, the aggregate tracking level was selected, as opposed to the usual unique tracking level. With aggregate tracking, only the total number of opens and clicks are recorded. This number is recorded for all instances of the action across all recipients. As an implementation detail, a unique total of 1 is displayed to show that the action is counted, but not to the level of providing unique totals. If the unique tracking level is used, then the unique totals count how many different recipients performed the action. The gross totals as usual count all instances of the action across all recipients.