Even bullets with a flatter trajectory such as a 338 Lapua SEEM to rise as the distance they fire gets longer. The earth curves down in any long distance shot and the bullet would stay the same. To combat this we use bullet drop compensators.

A bullet increases in speed and then suddenly decreases in speed but there is little/no (depending on the bullet) sustainment of a single speed as a bullet is not self propelled. This coupled with the spiraling effect of a bullet cause it to rise and fall- think of a perfect spiraling bullet pass from a quarterback- the ball rises and then drops to hit its target, presumably a receiver for the lions ;)

Bullets do not immediately rise after leaving the barrel, that it why you can BZO at 50yds and hit the same level target in the same spot at say 200yds but not at 175.

Not to mention, you are usually aiming your barrel slightly upward when you shoot, if you know it or not. Things like placement of your sight have a huge effect on this as well, you need to compensate for the distance between sight and barrel. If you use a higher velocity round you will have to compensate less.