Take a look at the trajectory chart above – most trajectory graphs look something like this, albeit typically less busy. Have you ever wondered why every trajectory is an arc? Why does every round start its trajectory off zero when it leaves the barrel?
The answer has to do with our old pal Gravity. The diagram below is a great example of what happens but it incorrectly uses the term ‘bore axis’. Bore axis describes the relationship between a users hand and the height of the bore on a given handgun. A higher bore axis (ex. on a revolver) results in increased muzzle flip while a lower bore axis (ex. on a Glock) results in less muzzle flip.
Now that we have the terminology gaffe addressed look at the image below. It’s easy to assume that the sight plane we establish on a rifle is perfectly flat. That’s not the case though- gravity is working on the projectile as soon as it leaves the barrel. To compensate for the inevitable projectile drop all sight planes factor an arc to hit the target at the desired range – which is achieved by creating a relationship between front and rear sights like you can see in the top half of the image below.
This is the same reason that adjusting your rear sight changes your zero yardage on an AR pattern rifle- moving that rear aperture up or down changes the POI to cross zero at the desired yardage.