Here is a bit from the mail -
"If you were waitlisted without an interview you showed potential as an applicant, but may have been weaker in one or two areas. By being put on the waitlist we are giving you a little more time to strengthen your application. We will contact candidates who have been waitlisted without an interview if we would like to set up an interview with them."
I'm not sure what it means for candidates who have had their interviews though but just quoting from the mail I received.