Technically in a scenario where one drops a bullet and coincindentally fires a rifle that is aimed horizontally, the fired bullet will impact the ground a very tiny amount of time later than the time it takes for the dropped bullet to hit the ground. This is due to the curvature of the planet.
Correct. Only The other way round as shooting perpendicular to the force of gravity (not parallel as the earth is curved as you said) means you are shooting UP in regard to the curvature. So the dropped bullet hits an infinitesimally instant sooner.
It could also be a function of terrain and topography.