With a higher rib, there is less barrel interfering with your field of vision on raising targets. This results in faster target acquisition and higher scores.
One of the surest ways of missing a target is to lose it under your barrel. When a target gets under your barrel, the natural tendency is to raise your head to get a better look (typically subconsciously) and you shoot over the target. A raised rib can help prevent this by reducing your blind spot a little.