There have been cases where a seller has stated that X silver is worth less than the conventional gold price, but these are few and far between.
Its just been a number we have stuck by and used for many years now, to the point where official auctions also state that the transfer rate for this auction is 15 silver = 1 gold. (just remembered this).
I cant remember how it came about, but its now been a standard so long people use it without question.