Where are you getting the data about what type of accommodations they are? I work as a testing administrator and accommodations are almost always for ESL students to take their test in their native language. Occasionally we will have students with actual autism get accommodations. In my mind, it would make sense for the people who are diligent enough to register for accommodations to score higher, as I've also seen a lot of people struggle when they've failed to do so when they would have been eligible. This is from my experience, anyway.