Because you bought a gold fund instead of gold. The characteristic of the fund is that there is a confirmation time for trading, which is generally 1-3 working days. The buying price and selling price are based on the time node at the moment when the buying or selling is finally executed (that is, 1-3 working days later), not on the time when you confirm the operation.

So you mentioned that you want to buy the price of 365, there is no way to do it, because you can't predict the price at the moment of purchase. So you either go to a formal platform (such as a bank APP) to buy paper gold, or buy physical gold, or accept this mode of operation of the fund.