Doverham,
You'll hear all kinds of reasons by people that are why an individual barrel or both barrels hit in a different location than aimed, but many will ignore the obvious and insist it's gun fit no matter the evidence.
Double guns have relatively crudely built and assembled barrels. They aren't beanfield rifles intended to hit point of aim hundreds of yards away, so they haven't developed manufacturing precision to that end. Heck, even a "best gun" from the best makers have barrels that are tied together with bailing wire, leveled with a Mark 1 eyeball and soldered with a torch.
Barrel tubes warp when bored and turned from the stress relief effect. Good makers will "hand straighten" using a little screw driven press made for the job. Straightness is checked by, you guessed it, Mark 1 eyeball.
Then there's the choke boring/openning up, after the barrel is bored on machinery. On double guns of the type we fawn over, it's often done by hand using reamers that are piloted, but skill is required. I've seen (recently) a gun of mine had the choke bored offcenter enought to cause a significant shift in POI.
The point is that barrels on our double guns have a lot of hand work and eyeball checking. So, opportunity to err is easy to find.
