Truth, and I will add that minor differences in ball size, in a smoothbore, does not make major accuracy differences as it will in a rifle. With a smoothbore, you can fire quite an undersized ball and just adjust your fit with thick patches, such as denim. If you "chew" the ball, you can go to a thinner patch. The road to accuracy in a smoothie is with trying different powder charges, (not being afraid to go up) patch thickness, (like a rifle) lubes, and some guns prefer bare-ball over patched ball. Playing around with wadding can make a big difference.
My Brown Bess, shoots best with a chewed .690" ball, a .020-.023" patch, one wax wad and one wool wad under the ball, and 140 grains of 1fg. Another one, exactly like it, might not like that. I have a .58" smooth bore that shoots most accurately with a chewed bare ball, wax and wool wad under the ball, and a wax-wad over the ball to hold the ball in. Go figure!!