[joezydeco]

This comment from mitrovarr@Metafilter sums it up:

"It's really easy to contaminate PCR reactions with human DNA, on account of the technicians being human and thoughtlessly shedding DNA at all times. You can prevent it, of course, but most labs that usually deal with non-human taxa only don't bother to, because it doesn't matter and it's a tremendous hassle and significant expense to do so. Unless this was done in a forensics lab, that would be my guess as to the source of the human DNA in the reactions."

http://www.metafilter.com/154105/Quality-meat-No-fillers-Or-...