Elevated shots go high because when you take the shot, you form a right triangle with the path of the bullet as the hypotenuse, the altitude (straight verticle line) from your elevation to the target as leg 1, and a line from your feet perpendicular to the altitude as leg 2.
Scopes are zeroed on level ground to compensate for the effect of gravity over the entire distance of a shot. When you shoot uphill, gravity only effects the bullet for the length of leg 2 of our triangle not the entire shot (hypotenuse).
As an example there is a rabbit 50 meters away from me (slant distance, aka hypotenuse of the triangle), and 30 meters above me (altitude, or leg 1). Leg 2 = 40 meters. If I take the shot compensating for 50 meters, I'll be high because gravity was only effecting the bullet for 40 meters, the length of leg 2.