This is just a thought and I do not offer it as the answer but for discussion.
Might it be that, like English double shotguns which are shot on game with the bird often apparently well clear vertically of the front bead, that it was the practice of the maker to make the gun shoot instintivly.
That is ( I am not luckly enough to own one of these masterpieces) if one takes the rifle concentrates your focus only on the target, bring up the gun and shoot without aiming, what is the fall of the bullet?
Now the problem with this is that these guns were made for the individual and fitted to their body and style of shooting so this might not be conclusive but I would be interested in the results.
I offer this as I shoot both shotguns and rifles with open sights with both eyes open. My lever action for example drives anyone else that uses it crazy as it hoots 4" off point of aim for them.

Regards