Found this on another site as the answer.
This just has to do with how horses back legs move when they gallop. If you watch slow-motion footage (or if you know a tidbit or two about horses), their hind legs don't go straight forward and back like their front legs do. The hind legs move in an oval pattern- More or less straight back on each stroke, but then sweeping outward in an arc away from the body before coming back in. As a result their hind legs travel a longer distance with each step.