Because there is a price difference between the on-exchange transaction price and the net value of the over-the-counter fund. The main basis for fund price changes is its net value. As long as the fund's net value is increasing, its price will rise. The ratio of the fund price to the fund's net value is called the premium rate.

The premium rate refers to the ratio of the difference between the unit market price and the net value of closed fund shares to the net value of fund shares. The formula for calculating the fund’s premium rate is: (on-market price – off-market net value)/off-market net value * 100%. Generally speaking, the closer the prices are, the lower the premium rate.