Autism Estimates in the UK and US - Why Such a Huge Difference?

The CDC has recently estimated that 1 in 150 persons in the US have an autism disorder. In the UK Simon Baron-Cohen states that the accepted prevalence rate is 1 in 100. Why is there a difference, a very substantial difference, between these two countries? Is the difference due to different medical and educational systems and different methods of reporting or is it due to environmental differences? I have not seen anyone explain this difference. If anyone knows of any explanations fore the huge differences in estimates between these two countries please feel free to offer your comments.