The stock index is equivalent to the real spot value, and the futures index price is what people expected at that time. The price of all commodities is not necessarily equal to the value (including spot), because the true value of most commodities is not easy to determine. Because people have different views, the expected value (forecast and guess) can't be the same, and the expected price can't be the same. Therefore, in order to better meet the purpose of buyers and sellers, it is necessary to rely on computers to match. As the transaction continues, the expectations of traders will change, so the price is constantly changing.