There are several common reasons why this might be the case.
HTML iframe element does not send referrer
An online ad is typically served into an HTML iframe element on a webpage. When a user clicks the ad, the landing page request is redirected through an iPromote web server.
The iframe attribute referrerpolicy in the originating webpage determines what information about the referrer is sent. The default setting would use an iPromote web server address as the referrer. But an iframe could be configured to send the originating webpage address instead.
For more information, see the W3C working draft on the subject.
Redirect causes referrer loss
When an ad served by iPromote is clicked, it is redirected through an iPromote server on to the specified landing page.
Sometimes the referrer value of the HTTP header of the landing page request is lost. If this value does not make it to the landing page web server, the web server has no way of knowing the source of the request.
Instead of attributing the request to a known source, a third-party analytics tool attributes the request as a direct visit, not a click. iPromote would count the click because it was routed through an iPromote server. Data between iPromote and the third-party tool would therefore be discrepant.