mkk41 gave you some really good info.
Once in a while a gun comes along that has .411" throats, and having a bullet that is just a bit bigger helps.
Most .41's are indeed very consitant at .410", but as with anything man made, there are always variations.
Some guns will just plain shoot better with slightly bigger bullets regardless of throat size.
If one happens to have a bore that mics .411" for example, and has throats that are also .411", a .410" bullet will never shoot up to the guns full potential, but having a .412" bullet solves everything.