I have gotten this same question for years, and long before I met my Ethiopian dh. I'm as white as they come, born and raised in the USA. But if they skip the step of assuming I'm Amish, people always ask me where I'm from, lol.
So, it might not be racially motivated nor malicious. Human beings have a tendency to say dumb stuff. Especially when trying to fill in a conversational gap or relate to people who are new to them. I've been guilty of that. So I try to be gracious in my answer and not assume malice or even stupidity when people say dumb stuff.