That's because whoever coded this page decided that it's a good idea to emulate "background-position: fixed" by updating the "background-position" of the #main element on every scroll event. Which seems quite stupid if you ask me.
It's probably done like that because iDevices don't support fixed positioning. Some browser detection and different CSS/JS profiles based on browser type would probably be warranted.
That's idiotic, sacrificing the majority for the minority.
I can see how mobile devices that don't want a mobile version of the site kind of necessitate this, but surely Apple could have gone about this better.
word. yep, something like background-attachment to fixed when the element is scrolled into view