Even though the report and smart list may be looking at exactly the same thing (e.g. an email sent to a specific group on a specific day), many times the numbers in the report do not match the results of the smart list.
Reports in Marketo like the Email Performance Report are returning results of activities that occurred in your instance, while smart lists return results of currently existing person records. When a person is deleted or merged, the activities associated with the person do not get pruned or purged.
So a smart list that looks for opened email will return all the currently existing records who have that open activity but will not return results of deleted or merged records. Conversely, a report looking at the same thing will return all activities, even if the records no longer exist.