Hear me out: if someone believes in horoscopes, they know what each sign mean. Which means they recognise themselves into a sign. Which means they will mostly act in ways to conform to that sign, giving you a way to figure that person out. The sign tells it all, it’s a self-fulfilling prophecy.
Is this unpopular? Astrology people are definitely influenced by “what they should be” and will regularly lean into or excuse their own negative personality traits because of their sign. “Yah I’m unreasonably stubborn, but I’m a taurus what can you expect teehee”
They’ll also project expectations and traits onto others before they even know them, if they get a hint of their sign.