When looking at your Email Distribution report, you will see a metric on the left pain of the screen, "Unique Opens".
The use of the term "unique" is referring to the recipients viewing settings within their email. If the recipient has their email setup in a way where they will have access to a viewing preview. Agility will NOT identify this as an open, the recipient would have to go one step further and click on the email to open it. This action would then be accounted for in the "unique opens" reporting.