I can stay in the saddle when/if my mount jumps over a small log or a ditch on the trail. But I can't "jump". There's a difference between my kind of jumping and going out into the arena or cross country and taking one jump, then another, then around a corner, then two in a row, etc. like jumpers do.
In order to do that, they put in HOURS and HOURS on the flat making their seat secure and their legs really strong. Those that don't do that as a foundation, are really just getting lucky when they jump, and we all know that luck always runs out sometime.