In Graeme Wright's book he discusses how a double can come together at a velocity below that for which it was regulated, then divirge and come together again at the correct velocity.

It happens in my rifle with very light loads with full weight bullets. I suspect that, at least for my rifle, there isn't enough recoil to "uncross" the barrels. I beleive that my theory is supported by the fact that the rifle shoots low with these light loads as well, indicating the there also isn't enough recoil to create sufficient muzzle rise. Faster loads with more recoil conform "to the rules" and more velocity brings the barrels together, vertical change doesn't seem to be much of an issue past a certain velocity and recoil level. But I haven't taken a bullet and powder selection through the whole progression that Wright describes.

JPK