I created a second profile with an include custom filter for a specific referring domain. The weird thing is that the data regard that referring site is different in terms of visitors, time on site, etc. between the two profiles (the general one and the designated one with the filter). Why the difference?

Rating: +0

When segmenting by referral, it is actually better to use the "Campaign Source" for the "Filter Field" instead of the "Referral". The referral is not a value that is held throughout the session. It only applies to the page the visitor lands on. If a visitor lands on page-a.html and then goes to page-b.html there is no referrer for page-b.html as far as Google Analytics is concerned--so no data will be recorded. You'll notice that in a profile segmenting purely by referral, the "Pages/Visit" column in the reports will all be around 1 page-view per visit.

If undefined, the "Campaign Source" is automatically set to the "Referral". This value is held throughout the visitor session.