If the PS pump pulley comes off something is seriously wrong, that thing is a solid shaft with a tight press fit . No worries there guys.

 

I would guess allot of it has to do with the offset between when the water pump bearing is an where the belt rides. The belt rides a decent amount rear of the bearing. The 8 rib belts are not as forgiving when it comes to stretch, they don't stretch near as much as a 6 rib.