I suppose that is only part of the equation and doesn't explain the day-to-day fluctuation in the world money market. Like many other things the value of one country's currency is what other countries are willing to pay for it on the various stock exchanges around the world.
My equation was long term.
Short term variations are caused by interest variations, government changing the interest rate, hirings and firings, and world markets. But those cause blips, not long-term changes.