A Smart List created to track Opened Email activity does not match the Opened count on an Email Performance report
Smart Lists specifically track "Open Email" activities in the activity log. "Open Email" is logged when a single-pixel tracking image is downloaded into the email, but some email clients do not download images by default, so it is technically possible for an email to record a clicked link activity without logging an open. Email Performance Reports have a backfill logic to compensate for this.
Rule #1: Each email activity record is set to one, and only one, of the following: Delivered, Hard Bounced, Soft Bounced, or Pending.
Rule #2: If the email record shows Opened, then it is counted as Delivered.
Rule #3: If the email record shows Clicked Email or Unsubscribed, then it is Opened and therefore counted as Delivered.
Rule #4: If the email is Opened, then bounces are ignored. If the email has not been opened, Hard Bounced "wins" if the email record also shows Soft Bounced or Delivered.
NOTE: As a result of this logic , a metric could potentially count an event (e.g. Delivered) that doesn’t have a corresponding activity log item. This can cause discrepancies in the results when compared to smart lists using email filters.