Your AV program is only good as the latest Virus definitions. It doesn't matter if you have Norton 2002, 2003, or 2004, they all get the same definitions when you do the live update.
Also, the discrepencies in numbers of virus detected could be how each company defines a virus too. I'd think the the "virii" in the zipped file are reletively old and already known stuff. And I noticed that McAffee likes to count some adware as virus while Norton does not and vice versa.
So basically, I'm thinking this test is teh stupid.