Originally Posted by Clark
The eye does not stay lined up with the scope tube, but with the erector.
OK, I'll put on my optical hat.
The erector tube's optical axis is rarely aligned with the scope tube's axis. As that tube is cocked at some angle off the scope's tube (and objective lens optical) axis so the desired aiming point in the target's image is centered on the reticule. Nothing behind the eyepiece ever gets aligned with it.
Fact is, the eyepiece optical axis is fixed in place on reticule centers when the reticule is between the eyepiece lens and erector/zoom lenses. Therefore, the aiming eye is in line with the eyepiece optical axis that also goes through the reticule center in such scopes. Otherwise, reticules so positioned at this place would never stay centered in the field of view.