If you want all your shots to be consistent, you either fire from a clean barrel (cleaning between shots) or shoot a fouling shot or shots, then fire your group - at least with modern, non-corrosive ammo. The problem lies in that much of the ammo used during WWII by some countries was corrosive. While the powder itself might be non-corrosive, the primers were. The compounds used to ignite the powder charge in the cartridge case left metallic salts in the bore, which are highly agroscopic - they attract moisture (water) and promote rust in the barrel. If you're in constant action it's not as much of an issue, but any good soldier would clean his rifle as soon as he had a few minutes of relative peace and quiet.
However, the assumption of that magazine is that a sniper wouldn't be aware of the clean bore vs. dirty bore difference in shot placement. I can't believe that anyone competent enough to be assigned as a sniper wouldn't have at least a basic understanding of such a simple principle of shooting. Any "sniper" that didn't know the principle obviously never spent much time practicing with his rifle and deserved what he got. I bet they wouldn't have checked the accuracy between different lots of ammo in their rifles, either.