Why does a 44 black powder pistol shoot 451 and 454?

A black powder pistol (single shot) in 44 caliber should take a .420 - 435 ball, A revolver on the other hand will take a .451 - 454 ball, the reason being when you load the cylinder of the revolver with a larger ball than the bore, you will shave a small ring when you ram the ball over the powder, thus creating a tight seal that prevents the gases from escaping around the ball and lessening the chance of a crossfire (several chambers going off at once) I have included a link to a pictorial guide that can explain far better than I can.