What explains the differences in testing on different antivirus comparison sites?

Question

There's a whole range of antivirus testing sites on the web. Some are obviously linked to a certain brand, others far less obvious. I found a range of sites, all giving a rather different view on some of the big ones. They also differ in tested programse (noteably on Vipre, the one I've been looking into but didn't buy yet because of the lack of testing information and contradictory results).

What could explain the differences in "objective" testing of AV/firewall applications? Which sites would you trust / trust not and why?

Some sites (feel free to add your own):

http://www.av-comparatives.org/

http://www.av-test.org/ (I guess the one I trust most, don't ask me why)

http://www.antivirusware.com/

http://www.anti-malware-test.com/

http://www.pcantivirusreviews.com/

magazines :

http://www.pcworld.com/reviews/collection/5927/2011_paid_av.html

http://www.pcmag.com/article2/0,2817,2372364,00.asp

Answer 1

My guess is that most of these "tests" are run by marketing organizations, not testing organizations.

I don't know how you'd actually test AV objectively, though - even one that recognizes 100% of known threats would fail on the next new threat. And if there was an accepted objective test, then eventually the products would be designed to pass that test, and not necessarily to protect you.