There could be any number of reasons a manufacturer recomends a tire presure. Remember the Firestone suv tires that were "bad" turns out ford recomended a preasure too low for that tire on that car. At least thats what I hear. They could recomend it based on load, noise, comfort, or any number of things.

My hydroponics experiment

You ever notice how hard it is to lip read cartoon characters?
"Crazy is what the sane call Delta Flyer"