Astrology has a great test-retest reliability. Every time someone tests your sign, the same answer comes out. That's what it means. Same for personality inventories: people's replies don't vary much over time. As astrology shows, that doesn't mean you can draw conclusions.
But star sign doesn't really correlate with personality traits. People with similar horoscopes can have vastly different personalities. People with similar Big 5 scores have similar personalities.
Let's just say that if astrology was reliable, people would follow their horoscopes.