Why don't picatinny optic mounts line up scopes perfectly?

[superlative]

When I mount a Leupold rifle scope picatinny mount in front of an Aimpoint M4 AD quick release mount in front of a Hensoldt RSA-S riser mount - the scopes don't line up. They are off to the side of each other.

I would think the whole point of a standard rail like picatinny is so that anything mounted to it would be centred in parallel with the rail. Why is this not the case?

Right now, my Leupold mounted 1-6.5 scope is on the rifle, and I thought I'd shortcut the sighting in of an Aimpoint M4 and the Hensoldt RSA-S by placing it in front of the 1-6.5 scope and lining up the dot with the reticule, but in doing so I noticed that both the Aimpoint and Hensoldt do not line up with the Leupold mounted scope, and neither do the Aimpoint and Hensoldt line up with each other. I can't for the life of me think of a reason why manufacturing tolerances would be so poor for these three reputable brands!

 

[TAP375]

I believe Picatinny standard refers to the rail dimensions only. Different manufacturers' rings may be slightly offset to one side or another. As long as the offset is on the same side, the accessory will be parallel to the rail - but not necessarily vertically aligned over the bore, as you found out. No big deal. In fact, some are offset intentionally to the side opposite of the bolt handle to provide more clearance for its operation.

 

[Silverado]

I think it's safe to assume that a quality rail will be consistent throughout its length. The answer is in the rings/mounts. If all your optics were mounted with high-quality matching rings, and all fasteners torqued to the same value, I suspect this would resolve itself? (Lapping may be required to true the rings, but shouldn't on really good ones.)

Do you have a set of alignment bars to check the ADM and the RSA mounts for trueness to each other?

 

[superlative]

I don't think alignment bars will yield much results. The scopes look parallel with the rail, it's just that they're not vertically aligned with the bore. And one in front of another, the ADM and RSA mounts look parallel - they are just not vertically aligned.

Aimpoint M4 mount I have:

Hensoldt RSA-S mount I have:

This is the Leupold mount I have:

(images are not mine)

I do have a absolute co-witness riser for the Hensoldt, but that riser is unique to the Hensoldt with a very unusual height that is not made anywhere else by anyone - in other words, I'm stuck with it if I wanted absolute cowitness.