The stab's profile is curved. The stab as constructed right now is not. The stab is built up from flat pieces and right now is flat.

If you look at the plans, it shows the stab thickness tapering from it's point of maximum thickness. If the elevator were hinged right now with it's centerline matched up to the centerline of the stab, there would be an equal difference top and bottom, and it wouldn't be very much difference. The idea is to sand the shape of the stab to flow into the shape of the elevator both top and bottom. That's how they planned for you to get the symmetrically airfoiled shape.