Why do some subscribers have a very high open count?

Opens are counted every time someone clicks a link or downloads the images for the email message.  If they download the images and then toggle back and forth between another email, each toggle will count as an open.  Also, the link tracking and image tracking used in the open count is unique to each email sent.  So if they forward the email to other people, each open and click to the forwarded email will count as a open and click for the original subscriber who forwarded the email.  

To see where the opens came from for a specific subscriber:

• View the Email Tracking “Who Opened” tab and sort by email address
• Find the opens by that email address and look at the IP Address of the computer that opened the email and the date of the open
• If there are multiple IP addresses and times of opens, the person most likely forwarded the email onto other people who then opened the email
• If there is one IP address and similar times of open, they probably toggled between the email and other emails
• Also, view the "Who Clicked" tab to see if that email address clicked different links from the message