It seems like if there is an advantage to an atlatl, it'd be range? Gravity should affect both the same, but it not having the same effect makes me think the dart has fletching that the spear doesn't. Aerodynamics is usually what changes how fast objects fall, meaning the dart is designed to stay in the air more than the spear.

[–] hallettj@leminal.space 11 points 3 days ago

From the paper,

Launching a dart via an atlatl ‘normally’ requires that force is applied by hand to the short arm of a lever, moving the dart at the long arm of the lever [...] a downward launch of an atlatl dart may partially hinder or entirely deactivate the biomechanics required for the atlatl to work optimally. [...] [Additionally] the atlatl dart’s light weight may result in it more easily rotating in mid-air such that it is no longer perpendicular to the ground.

My guess is that the dart falls out of the launcher fast enough to miss some of the push from the launcher.