UPDATE: If I put in my Report Smart List "Only include those that HAVE NOT OPENED Issue 1, then the number of "Leads that Opened Issue 1" is the discreapancy difference.
So I guess this boils down to "How are leads that did NOT open an email getting counted as Opened in the Email Performance Report?"
A smart list pulls all leads who opened an email, where as an email performance report tells you how many opens occurred. The opens in the email performance report are not necessarily unique.
I figured that was what it was Tara, even though the Performance Report CLAIMS to be simply a lead number, not an Open Count.
And then when I found that asking the report to include those that DID NOT Open the email, that it's some really weird counting problem.
AND when I run a Smart list for Leads that have Opened at least twice, the number was zero.
There are some detailed threads on the differences between the smart list and the email performance reports. Please do a search for those articles as they may shed more light on the issue.
Email activities are complex.
The "correct" count you should rely on is the one that is in the email performance report (it also matches the summary page of the smart campaign). The reason the counts will differ is because business logic is added on top of the email performance report. The smart list is pulling a raw list of leads for which that activity occured. For example, many times a “soft bounce” can occur after a delivered event. This means that the delivered count could initially look higher, then slowly decrease (it may decrease steadily over time) as we receive bounces from mail servers. It's a weird concept to grasp, but that is the order in which these soft bounces are reported to the sender. As we process all of these activities, we update the email performance report accordingly. However, if you were to run a smart list of "was delivered email," it would still include the folks that had a "delivered" event logged to them (even if there was a bounce shortly after).
This is just one example of why the counts will be different...there are many more nuances similar to the one I just mentioned. I recommend using data from the email performance report if you're trying to understand your mailings.
For the Email Performance Report, an Open will be backfilled if a Click activity is logged. Opens are determined by whether or not a lead downloads images from an email or not, so it is quite common for a click activity to occur, even if an open activity never does. The Smart List does not do the same backfilling, so Opened Email only looks for records which have an associated Open activity, not just an implied open.