Dealing with those who don't believe you have AS
Sometimes people think there being nice when they say things like that. They presume everyone including you sees AS as a bad thing. They think it's kind of like telling someone, "Oh you're not overweight. You're fine!"
Other people have myths about what they think people with AS are like. It is pretty common for people to think that if you're nice and have consideration for others - you can't possibly have AS because people with AS are absolutely self-centered and are completely incapable of thinking of anyone except themselves. There are even many clinician who believe things like that.
Others have a very high threshold for what they think someone with AS must be like. Many people including myself learned to make eye contact and learned to engage in informal social chit-chat. People don't necessarily see the struggle you went through to become less socially impaired or how exhausting it can be to act normal.
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"Everyone is a genius. But if you judge a fish by its ability to climb a tree it will live its whole life believing that it is stupid."
- Albert Einstein
Is it possible that their reaction driven by ignorance, eg they believe in their ignorance that to be autistic you have to be like Rainman, who wasn't autistic anyway. You seem nothing like that to them, so they think you making a droll joke or something? I think we here at WP sometimes forget the magnitude of the false stereotypes and ignorance in the general population.
Not that people don' t believe every autistic is like Raymond (aka Rain man), but the official movie synopsis does say he is autistic which is why people believe that every autistic is like him. It's the only tangible reference they have to go by.