Email Click Reporting - Calculation Question

I am looking for information on how Marketo calculates email clicks. 

In the email Program Dashboard - Summary, Marketo reports the overall number of clicks the email received.  However, when I generate an Email Link Performance report, the number of clicks is significantly different, almost always higher.  Is the dashboard number unique clicks or some other metric?

I have been unable to find calculation information in the documentation - apologies if this has already been addressed somewhere.