People who believe in one superstition are far more likely to believe another. The ones who don't are those who think belief in such things is magic, witchcraft or just contrary to the teachings of Christianity. In other words, their primary superstition, not logic and reason, causes their disbelief.
I think astrologist over a period of decades equipped birthdays with astronomical signs based on the calenders they were using with the ranges of similarities in personalities. Which is the only way they were able to match humans up with astronomical signs.