If you watch a dressage rider you'll see that they sit deep when cantering. Show jumpers however, come out of the saddle each stride, almost like posting. Why?
The reason has to do with the mechanics of jumping in the modern forward seat. When a horse jumps a large fence, the rider leans forward to go "with" the motion of the horse and free it to jump - this is what the forward seat was designed for.
In order to do that more easily, jumpers ride with shorter stirrups. This in turn makes it harder to sit the canter, so a lot of jumpers do the "posting" thing because it's simply more comfortable.