When you dial up for 600 yards (with 100y zero) your bullet has three paths. 1st path: is relative to the bore 2nd path: is relative to line of sight and 3rd path: relative to the plane of the earth (which for our purposes is flat). In his scenario the bullet drops away from the line of the bore due to gravity. The bullet travels (climbs) above the line of sight and climbs above the plane of the earth (assuming not shooting down hill) before it descends to impacts the target. So the bullet climbs or drops relative to whichever reference you use. So it's not that simple because you have three different planes you can reference. To insist that there is only one reference point is to dumb it down. Members of this forum should be mentally adept enough to understand the different dynamics discussed. Now I would not go out in the world to try to educate this point but here it should be given that we can all appreciate the subtleties of a multi dynamic physics discussion. So in your example how does the bullet not climb if there is a high point after the muzzle? Because the bullet elevated (climbed) above the plane of the earth to its highest point before beginning to drop back to earth.