You a projecting modern commodity trading exchange concepts onto history - until the advent of "virtual" trading of gold - there DEFINITELY was fluctuations of the value of gold and silver between countries.

But at the time, not all gold coins were the same. A florin from northern italy was not necessarily worth the same as a ducat of identical weight. Different mints formulated their metal differently and counterfeiting was a real and serious problem. The rumour that a king had ordered the adulteration of the currency could cause prices to swing wildly. Asset backed currencies were not some edenic pre-lapsarian device of peace and plenty. As an economic mechanism they have serious problems; in some respects more so than a fiat currency.