It's very possible that pieces and coins are the main variables in quantifying their currency. So something along the lines of:
100 silver pieces = 1 gold piece
100 gold pieces = 1 silver coin
100 silver coins = 1 gold coin
Maybe not those exact conversions, but just as a guideline for how the terms work.
Personally, I'd be down to see silver be more valuable than gold in an alternate world. Why must the value of precious metals also transfer perfectly across time and space? This is a fantasy world. The economy can work however the writer wants.