It's a good question. Might it be because some are just radically not interested in how they look, what they wear because so much time is spent on special interest subjects etc and also some spend a lot of time in their heads, which leaves less time for self observation or comparison? These are just off the cuff speculations and no criticism is implied. I also think some people with AS have very good awareness of how they are and what they do.
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I have traveled extensively in Concord (Thoreau)