Maybe because with cname (alias), as the person bounces around the site, it registers as a hit for the alaised domain, not the actual website domain...
And with frame forwarding... it registers as a hit when a link is clicked, but once it gets to the site (even though its frame forwarded and the forwarding domain is in the browser) as they click around the site it registers a hit for the actual website name, since that is what's in the frame window.
Is that it?