It's all about combined center of gravity. in order to keep your shins parrallel and not fall over, you must also keep the ccog directly above your feet. your ass, hams, and back weigh a lot, so you need to be bent signifigantly forward your knees to balance. Once you add more weight to the bar, then the bar need only be a fraction of the distance "over the knees" to compensate for the back end of your body. If that made sense...