You defined a currency as a storage of value - that's incorrect. A currency has a fixed value, one US dollar = another US dollar.
WRONG
currencies value does change
the biggest currency trading platform (forex) always moves the price
MONEY is suppose to be the most stable where it should only meander by a couple percent per year.. but currency can vary per community
EG native americans centuries ago trading seashells near the coast buy them cheap but can charge a premium to another community far more inland
currencies are more negotiable and can vary for multiple reasons