Why would someone address me as ma’am?

I feel conflicted about this because the mailman in his late 20’s/early 30’s addressed me as miss but at cvs a girl in her early/ mid 20’s addressed me as ma’am. I understand I’m not 21 anymore but I don’t think I should be called ma’am.

I look older than 38
They’re just being respectful
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Why would someone address me as ma’am?
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