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First, that's not true, technically speaking. The price of the future is whatever the market clears at. Arbitrage is a strong force that keeps the future and the underlying prices in a certain relationship, true, but only under certain (though common) conditions.
Second, here we are talking about NGDP futures and with them specifically there is no arbitrage against the underlying because the underlying is just an economic number that you cannot buy and warehouse. So in this particular case the price of the future is purely prediction-based.