Because the results of the negative studies are zero-zero. That couldn't be the case if it were just a cohort issue.
The only explanation that fits with all the facts is collection and processing. And it is, in fact, the explanation that Mikovitz and (blood working group guy, forget name) said they have basically figured out.
By my count we have three facts locked down,
1) HMRV are infecting humans
2) the issues with detection have been collection and processing related
3) HMRV are detectable in much larger numbers in PWC.