The sky moved. The horoscope did not.
Astrology's ruleYour sign is the constellation the sun stood in when you were born.
The Earth's axis wobbles like a spinning top, tracing one slow circle about every 26,000 years. This is called axial precession, and it drags the whole backdrop of stars along the calendar.
The horoscope dates were fixed in Ptolemy's era. Since then the stars have slipped about 26 degrees, close to a full month. If your horoscope says Sagittarius, the sun was almost certainly in Scorpius or Ophiuchus. Western astrology quietly solved this by cutting its signs loose from the stars they are named after and pinning them to the seasons instead.
Drift rate: 50.3 arcseconds per year, about 1 day every 72 years.