The acceleration of gravity is certainly always the same, since it is a physical and mathematical constant. A bullet free to fall, subjected to the acceleration of gravity for the same time, will fall the same amount. Drop is the same. The problem with conceptualizing it is in realizing the problem is the angle at which the drop occurs, not the amount of drop. The same error occurs in shooting down an identical slope, and to the same degree.
All guns shoot high when fired either up or down a steep slope.
As an example of the degree to which this can be a problem, that .530 ball at 1800 fps, sighted in for 100 yards on the level, but fired up or down a 65 degree slope at a target 100 yards away will shoot 5.6 inches high. At 200 yards, with the gun sighted for that range on the level, the error is 38 inches. Few of us are presented with the problem in our normal shooting situations. Remember, though, that it works for all guns at all steep angles, and that short ranges reduce but do not eliminate the error. Squirrel hunters aiming nearly straight up at 20-30 yards should factor it in, and more squirrels will wind up in the pot.