What you are understanding wrong is that it is the same for every currency. A currency is universal, its value is determined by what you get in exchange for it. If I can buy a car for 20.000 Euro than I can set a value for Euros. The Euro bill is just a paper. Same is for the bitcoin. If I can get a car for 1 Bitcoin than that is its value. The Bitcoin itself is just a number in a Bitcoin wallet.