Christine,
Our Help Article on Email Performance reports gives a little insight on this discrepancy. You can view it
here. The important part is right near the top:
The metrics in the Email Performance report includes the activity of all leads including those that have been deleted, which may cause a mismatch with the number of leads a Smart List returns. Adding a filter "Email is not empty" to the Smart List tab in the report will filter out deleted leads.
Essentially, Smart Lists only show existing leads, while reports show unsubscribe activity. Each serves their own purpose, and there's a solution if you want them to match.