Answer
The scale shows the magnitude of the normal force acting on you. As shown in Figure 5.25 (page 84), on an incline the normal force is less than mg.
For the two to be equal and opposite, you must be in equilibrium on a horizontal surface. The net force on you is zero. The 2 forces acting on you are mg, downward, and an equal and opposite support force, upward. Only then will the scale reading accurately report how hard gravity is pulling down on you.
