The classical double rifle has fixed sights. It may be equipped with flip up leaves for longer ranges, but each of these in itself is a fixed sight in the sense that it has no adjustment. When the rifle is regulated, the first part of the process is getting both barrels to shoot together. The second and lesser step is to adjust the sights so they are looking at the point of impact. Hence the sights are part of the regulation picture. If the sights are fully adjustable then this does not apply.

In a perfect world the side by side would have to be regulated only for horizontal dispersion and the over under would have to be regulated only for vertical dispersion. These are the adjustments provided on a rifle with adjustable regulation in only a single plane. Hopefully the rifle has been built with enough care that dispersion in the other plane will be minimal. The change in regulation due to change in load will occur mostly in the plane for which adjustment has been provided.

Of course it's not a perfect world ...