Yup my vet would be one of those....he said he has seen OFA good sent off to OVC come back not clear. Frankly, I have done OVC and Pennhip the last few times.Penn Hip is MUCH more accurate. But you don't find that many vets in all areas that can do it. One Ortho specialist vet I know thinks OFA is pretty much a joke but it beats nothing.
I did redo hips on one bitch when I got mixed results from OVC and Pennhip - and four days after doing the xrays she came into heat. We redid them and the vet was so surprised by the difference he sent them back to both and also OFA....I will post the results when I get them in on another thread. He said that they have been told about heat cycles causing differences but he's never seen it personally.