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There are different factors that can cause this, but the main reason is that not all email servers respond back to Marketo's servers that they email has been delivered. If Marketo is updated with an "Open Email" or "Link Clicked" activity then it will infer that the email was delivered, which is why the email will show as opened when looking at the email campaign results. But when you create a Smart List with the "Was Delivered Email" filter, you are ignoring the "Infered" delivered when someone opens an email (or clicks a link in the email).