One factor Thomas didn't mention is that while the shot charge was the same, the powder charge was not. Assuming the faster burning powder charge weighed less than the slower, that might have also had some impact. It's the total weight of the ejecta rather than just the shot charge that has to be factored into the formula. Obviously, shot makes up most of the weight, but it's not the only component contained in the hull.
Any additional powder mass would probably be lost in the variation of shot charge. I don't believe that can account for much. But it would be nice to see recoil actually quantified instead of being left to observer perception. Seems very odd that they would choose such a subjective "measure" as their dependent variable.