I would agree with sgurr's comments re tester/calibration errors.

I am an audiologist and perform a more detailed version of this test on newborn babies. Based on my experience, other possible explanations are the presence of a temporary or fluctuating hearing loss due to outer or middle ear infection or obstruction at the time of testing (I presume the testers rule this out prior to testing). Also, as I understand, dogs are tested with a click stimulus which incorporates a range of particular pitches. It's possible that the dogs hearing is impaired for the pitches tested but may not be so for higher or lower pitches of sound. This might explain the appearance that the dog hears something.