Bullet trajectory has a different flight path than what the shooter sees in their line of sight. Optics and iron sights are mounted above the barrel and there is an offset of about 2 inches on average. The straight line the shooter see in their line of sight, the bullet is going to have a line of departure which is straight out of the muzzle. The muzzle is always aimed upwards making the bullet travel in an arc.
At some point, the bullet trajectory and the shooters line of sight have to intersect which is at the zero. Since the bullet is at an upward flight path, the shooting position to the shooters zero, the bullet will continue upward and then fall down. The shooter max ordinate is the highest point that the bullet trajectory will go until gravity takes over and the bullet starts to drop.