When you apply your park brake on a slope, the natural camber flexibility of the rear axlw will cause the car to move a little bit. The wheels won't roll but the body will shift on the rear axle. Same thing happens when you hang the car on the parking pawl except that the wheels will roll a little depending on how far the parking pawl is from engagement plus a little wear. 6-8" sounds like considerable wear.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD