Scenario 1: Regularly false notification.
This is a known bug. Sometimes the cron times out before it fetches all the sites and so sends a false email. We are fixing this issue within the next few releases.
Scenario 2: Occasional false notification.
WordPress fetched the update after the email was sent - WordPress scans your site for updates every 4 hours. The InfiniteWP panel could have sent the email before WordPress picked the update. The next email will be true.