Did you know that one DID alter can have a measurable, documented medical condition that the others do not, including but not limited to asthma, allergies, and even diabetes?
Anyway, while fascinating, that has nothing whatsoever to do with the substance of this post but, ya know... ADHD.
I don't think anyone knows and there's surely a Nobel prize awaiting the person who does figure it out. My best guess as someone with virtually no education in the subject, other than the psych 101 course where I learned that, is that there's a deep connection to the placebo effect -- another thing we don't yet understand.
Edit: I should add that virtually every case of DID is a result of severe childhood trauma, usually sexual in nature. The reality is much more tragic than it is interesting.