There are two possible reasons for this:
- The email has been edited at some point to remove the link. The report will show all links in the email over its life, including links that have been changed or removed.
- The link exists in the text version of the email, or in dynamic content for some segments of the email.
There is no way to track down the first option, but looking at the code for the text or dynamic versions of the email can track down the second option.