Top websites is determined using the HTTP referer.  As a result the actual referring website (the site in which the ad appeared) cannot always be determined.  If the campaign is being run on Doubleclick Campaign Manager (DCM), the referring website cannot currently be reported.  It is important to note, however, that all clicks are captured irrespective of the referrer being captured.  As such, all clicks can be still be profiled at the campaign level.