One’s Zodiac sign depends on the time of the year when this person was born. At any particular location on the terrestrial orbit, the Earth is only exposed to some part of the night sky. The part of the universe which is exposed exerts energies that, when reach our planet, affect everything and everyone, even the permanent state of mind of human beings. Thus, if you are a Libra, you were born when the Sun was in Libra. So the Libra constellation and surrounding space was blocked by our big star. And the energies exerted from other constellations actually reached the Earth (and your brain? shaping your personality). 

I am probably dead wrong but it’s the only reasonable explanation I have.