With a high vent the clay will sit on the top of the barrel when you shoot. With the normal rib your barrel will have to "cover" the clay when you shoot. With the high rib you can watch the bird the entire way. With a low rib you have to pull the trigger when clay is out of site.
Thanks John, That is now confirmed cuz that was the other reason I was told at work, thinking it's a combination of the two. And because of the flight path of the target in trap shooting, it's not used in skeet shooting.