Quote from loufah:
The bid and ask change as people modify their orders. The last price only changes when there's an execution. People change their orders as conditions change, as frequently as several times a second, but an execution only occurs when two parties agree on a price, and there can be minutes or even hours between executions; during this time the bids and asks are still changing. On a future with frequent executions, such as ES, the last price rarely lies outside the bid-ask spread. On a future like YG, the last price can be several dollars away from the bid-ask because the last execution may have been several hours ago.
