Hmmm. Sky & Tel May 1995 issue has a letter on page 8 on this very topic. "stereoscopic cues are caused by abberations in the optics. ... ordinary oculars ... present a curved image to each eye. When the eyes are not perfectly centered over the exit pupils, the curved images are seen from slightly different points of perspective, resulting in a 'three-dimensional image.' The effect in most pronounced in the least complex eyepieces..." This was by a R.A. Buchroeder of Tucson.
What if we had three eyes?