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
For example, the URL http://www.siteB.fr/ becomes http://www.siteB.fr/?xtref=https://www.siteA.fr. This way, our tag will be able to detect the xtref that's in the query string.