Yeah Im no economics major, but I know a dollar is worth a dollar since I have been born, based on what the gov says and the financial banking industry and power of the dollar.
Putting aside inflation/deflation, to make your statement comparable to Bitcoin, you would need to compare the dollar to another form of currency. A dollar is a dollar, but is a dollar always equal to the same amount of Yen or Euro? No, it fluctuates.
Same with Bitcoin, a BTC is a BTC but it fluctuates in comparison to other currencies, such as the dollar.