Why do some people claim they can "always tell" if someone is trans, when they obviously can't?

Today someone actually said to my trans husband that they can always tell who is trans. But they said it about our cis friend who just is a woman with a strong jawline. Like, they said "she's trans, I can always tell" about a cis woman, to a trans man. Clocking both people wrong, to their faces.

Yet even when our friend told them she's not trans, they didn't believe her. And when my husband told them he was trans, they also didn't believe him! Just flat out refused to admit they can't really "always tell." It made them look even stupider.

Why do some people claim they can "always tell" if someone is trans, when they obviously can't?
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