196

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.