Simple explanation. We follow western astrology which is based on the Equinoxes. The beginning of the astrological year is at the spring Equinox. We're still the same sign! There are many more constellations than the 12 in the zodiac, and it's the planetary rulers of each sign that represent our sign's personality traits. (which is why that 13th sign they spoke of with no planetary ruler could not be a 13th sign.) I hope you're relieved. Happy Full Moon week!
ps here's an article that explains this in more detail