So, out of desperation, we called Joshua Akey, an assistant professor of genome sciences from the University of Washington.From, The Case of the Mysterious Puppy Solved (Sort Of)
Akey explained that canine DNA testing is still in its infancy, and the accuracy is dependent primarily on whether or not a particular test has "markers" for the breeds that may be present in your dog. Much of the info for these markers seems to have been provided to companies by the American Kennel Club and other dog-fancier organizations.
In some cases, breeds not recognized by the American Kennel Club aren't represented well in the tests. In other cases, a test may have better markers for some breeds (say Labs and goldens) than others (griffons and otterhounds).
Akey explained that it's virtually impossible to get these companies to reveal the genetic markers they're using — so as a result, it's almost impossible to say which tests are most accurate, and the accuracy may really vary from breed to breed. So ultimately, dog DNA tests put owners in the ballpark, but they're rarely conclusive.