To evaluate the price of a Bitcoin from the perspective of its value as a unit of exchange you need to take into account the volume of the transactions and the

To evaluate the price of a Bitcoin from the perspective of its value as a unit of exchange you need to take into account the volume of the transactions and the time currency is held between being paid and being spent. If there were $1T of Bitcoin transactions each year and the coins were held for 1 year in between, then yes, the total value of all circulating Bitcoin need to be $1T. If the coins are held on average for a week, you only need $20B worth of circulating Bitcoin. When you leave out that last number, you implicitly assume whatever interval you're talking about is the transaction duration, which is usually a bad assumption.