If open activity is being posted on a lead who has not opened the email, it is due to other recipients opening the email. These recipients can be either in the “To” or “CC” or “BCC” field of the email. The tracking link can only identify if an email is opened and not who has opened the same. 

For one on one email delivery, check the activity history for the same.

You can check for the other recipients in the email campaign summary.