I measured my Mini-14, M1 Carbine, M1 Garand, M1A, and Lee
Enfield Mark 4 No. 1. Interestingly, I found that all of them had a heel to
sight aperture distance of 14” regardless of the LOP with 2 exceptions, the Lee
Enfield has a heel to sight aperture distance of 13½” (figures, British) while
the Mini-14 with its stock rear sight has a heel to sight aperture distance of
15½”.
 







 
 
 
 
 

Interesting observation. The reason peep sights are so effective is that the eye naturally seeks out the point where the most light passes through the aperature, thereby giving the best sight picture. This is an automatic  reponse. It would seem to me that your eyes have become accustomed to a certain distance between your eye and the aperature. On the one rifle that did not offer that "ideal" sight picture, you unconsciously attempted to find said sight picture by adjusting your body/head to the point where you got the sight picture you are used to. Your decision to adjust the rifle to match all of your other rifles was the thing to do.

Just like when you are shooting a Highpower match. We would set our position, hold, foot position, etc close our eyes and bring the rifle up. If we were not aimed at the target we would shift our entire body position so that we were, as opposed to moving the rifle to point at the target.  Huge over simplification I know, but you did the same sort of thing by making the rifle fit you better. Great info.