The notchback's hold the cars rear shape better because of the design of the rear section. Small, stationary rear glass and rear trunk area promote a far more rigid platform and don't have near as much lateral flex. The large opening at the rear of the hatch promotes a lot of lateral body flex. There is literally a nearly 4'x4' square of unsupported open area there. Open your hatch, then jack up one corner of the rear of the car, the try closing it again. The striker actually shifts a few millimeters out of alignment because of the flex. Your hatch actually helps hold the shape of the rear section. This is one more reason I'm not a fan of fiberglass hatches on street cars. They just don't promote the strength that steel does.