From my research, ball sizes in the old days were based on the number of balls per pound of lead. For example: 20 = .615 dia, 30 = .537 dia, 31 = .532 dia, 40 = .488, 53 = .445 dia, 54 = .442 dia, 79 = .389 dia, 84 = .381 dia. the barrels of guns were made to accept these typical sizes and every gun was sold with a mold of the right size for the barrel.
Today, we make balls in nice even diameters and barrels the same way. This is result of the influence of cartridge/modern gun and ammunition makers which don't rely on cloth or leather patches to interface between the bullet and the barrel. If you were to take the +.015 dimension to any of the above a barrel maker today would laugh you out of his shop. They make .62, .54, .50, .45, .40 cal barrels not the odd diameters of old. This is very often the typical mistake made by museums when they size old guns. (using today's criteria not the criteria of old.