Here's why:

1. Lewis Structure: The central chlorine atom has three oxygen atoms bonded to it, and it carries a formal charge of +1. One oxygen atom has a single bond and a formal charge of -1, while the other two oxygen atoms have double bonds and a formal charge of 0.

2. Resonance: The double bonds can be shifted between the oxygen atoms. Since the double bonds are delocalized, we can draw four different structures that represent the actual structure of the chlorate ion.

Important Note: Resonance structures are not real, individual structures. They are just a way to represent the delocalization of electrons in a molecule. The actual structure of the chlorate ion is a hybrid of all four resonance structures.