VBAP predicts perceived source azimuth accurately but not personal elevation

Evidence

The equation estimates the perceived virtual source direction with relatively good accuracy. The confusion cone azimuth ϕcc is estimated to an accuracy of a few degrees in most cases. The perceived confusion cone elevation of a virtual source δcc is personal for each subject (Pulkki, 2001b), but it is typically perceived inside the loudspeaker triplet. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1899

Categorization

• QUESTION: Why is perceived elevation personal while azimuth is uniform across subjects? Worth chasing whether this reflects individual HRTF differences.