To put it bluntly, it is to calculate the profit and loss according to the daily settlement price, "retreat more and make up less". Daily equity changes are compared with the settlement price of the previous day.
Therefore, under the mark-to-market system, the average price of the position shown on the bill is the daily settlement price, so it is variable.
The other is "transaction hedging". If the "transaction hedging" system is adopted, then the average position price is the weighted average price of each opening price.