Why on some days is there a price gap on the S&P futures market. Meaning why is there a major difference in the closing price and opening price. Example on 2/18/2022 market closed at 4343.5 and on 2/22/2022 it opened at 4324.5 almost 20-point difference between close and open.
The mechanisms behind the determination of 'special' prices like 'close', 'open' etc are non-trivial and vary from security to security. For example, 'open' prices can be (but not always) the average from a special auction period, rather than just the mid snapped at certain time. To fully understand the origin of these prices, refer to the exchange's rulebook.