While it doesn't explain the whole difference, it's not surprising that Stanford has a higher rate. First: the more demanding the environment the more likely you are to find (got example) milder ADHD to impact your life. Second: the more well off you are or more access to resources you have, the more likely you are to actually care to get diagnosed. Third: stressful environment can actually cause serious issues, suddenly. For non-education reasons I suddenly gained panic attacks while I was at uni and they took years to go away.

I'm sure there are more things like that.