It is the vector. The direction of gravity is partially in the same direction as the bullet’s velocity. So it counters air resistance a bit and the bullet goes faster than it would if you shot horizontal. Shooting ‘up’ is the opposite. It is about the vector at which the gravitational force is offset from the direction of flight when you are shooting horizontal. So it deflects more.
Air is air, bullets do not speed up if shot downhill... All air resists bullets and slows them down. The effect of gravity has so little do with it other than in a theoretical lab setting, which we don’t actually shoot so in ignore it.
No bullet ever travels parallel to the earth anyway, except for maybe a microsecond if you level the bore line, Bullets are always dropping on a curved trajectory once they leave the bbl:, up in a vertical component to account for range, down once past mid range. Geesh. Applied physics, not theoretical.