Correct me if I am wrong Bit, it sounds like you are trying to say that the + or - that we see in ballistic charts are the height of the bullet in relation to the muzzle as a horizonal line. If so that is true however the degree that the muzzle must be elevated in relation to this horizonal line is directly related to the LINE OF SIGHT.
In order to defeat gravity the muzzle of the weapon must be elevated on the weapons center axis so as to toss the projectile at an "arch" so that it will cover the sighted in distance and intersect with the aiming point before gravity, which is constant, drags it to the ground. This can cause the projectile to actually start out below the line of sight and cross that line of sight and hit the aiming point when it falls back to earth and intersects with the line of sight again, which is, hopefully, your aiming point. The key word or phrase is gravity is constant. From the time the projectile clears the barrel it is being pulled to the ground at a constant rate.
Now knowing that your barrel is already elevated it makes sense that if you are shooting uphill then that elevation becomes even more great and can result in a shot that misses high. Works the same downhill. As Bit says "aim low". Shoot uphill and you are over-compensating for gravity, shoot downhill and you will help gravity with thousands of feet per second, hence the bullet never attempts to "drop" there by causing a shot that misses high.
Its all about "line of sight vs distance, Velocity and mass" that determines how much positive elevation the muzzle must have to provide the correct "arch" needed to defeat gravity and create a satisfactory intersect with the aiming point.
I think!! ?