I had conversation with a parent, not long ago, where I told her that I thought she was right, her daughter did appear to have a mild form of autism. She appreciated the confirmation of what she believed to be true. Doctors and educators who recently evaluated this little girl did not agree with us. "But, there is not a right or wrong on this question" I said. "Five years ago" I told her, "I would not have thought your daughter had autism." There is a changing understanding of the diagnosis that we used to call autism. A growing appreciation that girls with similar neurological challenges may look different than boys is only part of the evolution of understanding of what we now call Autism Spectrum Disorders.
For an interesting review of one study looking at diagnostic practices, look at this article: Social Factors May Drive the Spike in Autism Cases.