If you say the sandwich you are selling me is tuna, I don't think it is unreasonable to expect there to be tuna in the sandwich.
from the link in the OP:
"In their second amended proposed class-action complaint filed earlier this week in federal court in Northern California, Karen Dhanowa and Nilima Amin claim that out of 20 samples tested, 19 had no detectable tuna DNA sequences.
All 20, the complaint states, had detectable sequences of chicken DNA, 11 samples had detectable pork DNA and seven had detectable cattle DNA. The samples were tested at the Department of Ecology and Evolutionary Biology at the University of California, Los Angeles, the complaint says."