The decline is not like the flat bench. Even on a barbell decline, the movement works your lats as well as your chest, meaning most people may be able to move more weight in certain positions once they learn the movement.
Doing the same on a laying flat press is something I would expect to see from someone who can move around 405 on a barbell, give or take.
I have simply never seen someone who wasn't pretty damn big moving more than 4 plates a side on those machines...and I don't mean for one or two poor reps.
My frat brother is over 300lbs (bouncer) and moves 5 plates for 10 reps (could possibly do more but he stops there)...and I really want to see the kid who can do the same without the muscle to back it up.