I thought the DAT was graded based on how people did in the past on that particular test you are taking. So how hard ADA thinks the exam is is based on how people have done over the year.
If your dentist is right, I would predict that it is harder maybe in the summer. Since most people will want to take the DAT when they have the most amount of time in the summer. People will tend to score higher if they have a lot of time to dedicate to the exam.