As Cythialee pointed out some cylinders (chambers) are a tad big for .451 round balls. I recently purchased a new Pietta and measured the chamber mouths. Each came in at .449, except for one which was slightly out of round and measured .451 across the widest diameter. Not bad, but a concern I didn't expect. Since I follow the practice of only loading five, that one serves as the empty.
As long as pure lead is used, there isn't too much difference in pressure between loading .451 or .454 round balls. .457 round balls are sheering quite a bit of lead. The problem is exaggerated is the chamber mouths are rounded from wear or were rounded/crowned intentionally. Then a thin ring is not cut off, but the ball is swaged down into the smaller diameter chamber.