Why is it .999 pure silver and not 1.0 pure silver? Is that one-thousandth a sort of hedge against whatever impurities might have been in the forge that day?
It’s very hard to totally purify any material to absolute purity (as much as Smirnoff would have you believe otherwise!) with a few exceptions. By the time you reach 0.999 it becomes less and less economically feasible to go any further, although it is possible, since you can buy 99.99999% pure silver also if you really need it.