Why do people think I'm an only child?

I'm not but people that first meet me always assume I am.

i am the youngest out of 3. my brother is 6 years older, and my sister is 8 years older. I'm really close with my sister.

but when I'm on my own and meeting new people like at work or school, they assume I'm an only child.

why could that be? are there certain traits that stereotypical only children have? I know I'm a little odd but I don't know what is it that makes people think I'm an only child. should I be offended?
Why do people think I'm an only child?
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