The Wikipedia language suggests the existence of perchlorates (discovered in other Martian soil in 2008) could have destroyed organics during the 1976 test heating, thus explaining some of the
negative results from Viking – leaving the
positive 'Labeled Release' result more credible. That's the interpretation of the paper described at:
http://www.sciencedaily.com/releases/2010/09/100904081050.ht...
That article is currently footnote #28 of the Wikipedia article, describing the 2010 paper referenced as footnote #25. Another footnoted reference from 2011 (#29) disagrees. But presumably, all that perchlorate-related reasoning was available to the authors of the 2012 paper defending the positive result.