Please see a broker's explanation below:
"When a product closes for the day and goes to a pre-open state, we carry the position forward at the settlement price (in electronic futures, we use the last traded price since the settlement is actually done by the pit and we have no access to that info until the end of the day.) When ZM closed at 1:15 then went from 'closed' to 'pre-open' a few minutes later, the open pnl was booked (from 320.1 to 319.3) and set to 0. The new avg price was set to the settlement of 319.3. That is technically correct and what to expect for all futures contracts. You may get the same question from a trader that sells the ES at 2:00pm at a price of 920.00 only to see his avg price change to 907.00 at 3:20 when the market reopens. It is called marked-to-market and is how all futures will be handled."
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