Further fueling the drama and confusion, the actual zodiac signs ARE named after the constellations. However, the zodiac signs follow 12 fixed segments of the zodiac band pictured above.
In second-century Alexandria, the great mathematician and astronomer/astrologer Ptolemy created the Tropical Zodiac, which is a fixed system that is not affected by changes in the constellations or the Earth's axis. Ptolemy used the same names for the zodiac signs as he did for the constellations, which is why there is confusion around the birth date ranges.
The Tropical Zodiac is static and not affected by shifts in the Earth's axis. It follows the procession of the Equinoxes and the seasons. The Tropical Zodiac begins every year with the Aries pseudo-constellation, which is based on the position of the Sun at the Spring Equinox on March 21. Western astrology follows these zodiac sign dates, which correspond to the Tropical Zodiac.
In ancient Babylonian times, when the 12 zodiac signs were first named, they DID line up with the physical constellations. But Ptolemy updated this system and gave us the one we use now. So even though the constellations have shifted, the zodiac signs haven't.
Fun fact: The word zodiac comes from the Greek term zodiakos kuklos, which means "circle of animals," referring to the animals that symbolize the astrology star signs in the Tropical Zodiac.
Bottom line: The zodiac signs and the constellations are two entirely different things.