Page "Head-related transfer function" Paragraph 6

The HRTF can also be described as the modifications to a sound from a direction in free air to the sound as it arrives at the eardrum.

These modifications include the shape of the listener's outer ear, the shape of the listener's head and body, the acoustical characteristics of the space in which the sound is played, and so on.

All these characteristics will influence how ( or whether ) a listener can accurately tell what direction a sound is coming from.