When site A uses the HTTPS protocol, and site B uses HTTP, the referrer is not passed along when going from site A to site B. As a result, the source of the visit to site B will be Direct access.
If you wish to know your site's referrers despite the difference in protocol, here's what we recommend:
Use of our xtor campaign tracking
Use of the xtref parameter on redirect links; it forces communication of the referrer to the site