Is it normal for one image in a stereo pair to appear a bit darker than the other? Is the small spacing between the lenses really enough to show a difference in illumination due to the (very slight) difference in angles?

I'd purchased a very nice Realist 45 on ebay. Despite it probably only ever having a few rolls through it, I did notice the right and left image appear to have a slight difference in EI (if I'm using the term correctly). Since the unit is from the 50s, I figured it would need a CLA anyway. The difference does not really cause any problems; it is not really noticeable when viewing the images correctly - but I know that doesn't mean anything.

Last week my best friend was given her grandfather's Corte-Scope (the earlier American Corte-Scope Company version). I noticed the same apparent issue on the cards that came with the Corte-Scope. Then I searched online, and noticed the same effect on many stereo pairs, both old and new. However, I could not find any information on this particular issue.