Why are hand clamp faces not parallel when not under load?

The jaws used on our clamps are plastic and are designed to flex as the load is increased. This is the mechanism by which clamp pressure is held. As these flex, the angle between the faces will increase. To combat this, we "pre-load" the faces by setting them 2 degrees in the opposite direction. This way, when a nominal load is placed on the clamp (1/2 of the rated load), they end up sitting roughly parallel to each other. In other words, if the faces were parallel before the clamp is used, the faces would end up non-parallel during use.