I really don't know. I never really checked it, but I always put a few rounds, up to a half dozen, through a clean barrel before I use it. I clean a bore when I can no longer see though it
Seriously, I think another bullet going through a fouled bore takes the excess crud with it, and the remaining junk forms a tight seal between barrel and bullet....therefore, accuracy. I would think, a totally clean bore would have some blow by and other factors, perhaps some wobble that would mess with accuracy. I'm a firm believer of being lazy. With my muzzle loaders, when I clean them, I put a couple of rounds through them for fouling and then make sure the breech plug, nipple or pan etc is clean.
Pete