The Market Center open can differ from the first reportable trade on a contract at market open, sometimes by several minutes. The opening price refers to the first reported trade from the primary exchange of which the contract is listed on. This differs from the first occurring trade that may occur across any exchange the contract is tradable on once the market has opened.
In order to see the Open price for the primary exchange of any given contract, add a ticker symbol directed to its primary exchange, for example IBM at NYSE or NVDA at NASDAQ and examine via the Time & Sales tool.
Conversely, in order to see the first trade after market open, query the Time & Sales tool via a SMART directed ticker symbol which will display the first reportable trade across any given exchange at market open.