The reason why you see the price of your option position change after the market closes, is because we have to load a different price for the product at that time of day.
During the trading day, we display the prices of options, as the mid-price between the best bid and the best ask. When the exchange closes, there are no quotes available to determine the mid-price. That is why we display the price of the last trade in the particular option instead. If the mid-price is different from the closing price, this reloading will cause the value of your position to change.
Impact on ‘Available to trade’ value
Since the exchange is closed when we display the last trade of the day, the difference in value does not impact the Available to trade value of the portfolio. Before the market opens on the next trading day, the close price will already be loaded.