Other people have already addressed this misconception, but I'll repost my old diagram that illustrates what's happening.
The bullet does not "climb" above its initial line of trajectory, that is impossible and would violate basic physics. Assuming your rifle and the target are on the exact same plane, your rifle is actually slightly canted upward, which means the bullet flies in an arc. If the rifle was held perfectly level, the bullet would drop immediately upon leaving the barrel. See the diagram below for a graphic illustration. Note the angles are exaggerated for effect.


However, your line of sight through your optic, or irons, is a perfectly straight line. So, since the bullet begins dropping the instant it leaves the barrel, and the rifle is angled slightly upwards, there are two points where the bullet's flight path crosses your line of sight. This effect is exaggerated on AR-pattern rifles since the optic/sights are higher above the bore than on other types of rifles.