In the center of this clip is a ParticleEmitter with a SpreadAngle of (180,180). The green line is the EmissionDirection and the red line is a bias in the spread (notice that there are a lot more particles near it’s ends than everywhere else).
https://puu.sh/yF0MD/740c2d2dee.mp4
In this case I’d expect the direction to be completely random.
Repro
SpreadBug.rbxl (14.7 KB)
This is just the visualizer I made for the clip + a version without it.