I think that the rides that a lot of these top level hunters give actually DO throw you out of the tack pretty easily. A lot of the riders aren't trying to make it LOOK like they're getting popped out of the tack, they really ARE getting popped out because their horses can crack their backs so hard. I mean, look at Popeye K...he jumps sooo round that Tommy Serio had to put Sadl-Tite on all of his tack...his boots, saddle, reins, gloves.
Also, I have a friend whose horse is VERY nice, extremely competitive, jumps GREAT and due to his conformation and style when he jumps over a 3'6 oxer his head and neck drop so low that it DISAPPEARS and there's nothing left in front of you. Because of that, she tends to stand on her toes and duck because she basically only has the back half of the horse there.
And then there is that style. A lot of hunter riders ride in that forward seat, albeit not all of them are effective at the same time (like me!). The wide hands, long reins, all of it is essentially trying to create an image of fluidity. It's just how a lot of people ride now.
If you consider the juniors who will go around ducking in the hunters and perching perfectly in the eqs, that's a completely different ride. For one, half of them have different horses to do the eqs, but also, in the hunters you are trying to make your horse produce a beautiful, smooth jump. That causes many people to ride differently, trying to do whatever they can to make their horse's round seamless. Although ducking isn't the way to do that, sometimes it happens when you're trying to open up your horse's stride and get a nice gappy distance. It can be distracting, but sometimes it just happens. In the medals and stuff though, the goal is to maintain equitation and essentially be perfect. If your horse ends up getting a little tight to a jump and drops his knees a bit, as long as you maintain your position then that isn't going to be the same kind of penalty as it is in a hunter class.
Ideally, all of these things could be done maintaining the same, perfect position, but we don't all ride like George Morris, unfortunately. It is what it is.