It’s entirely up to the entity quoting it. That could be me, for instance, putting a bid into the market because I read a hot tip on HN. Or you may prefer to

It’s entirely up to the entity quoting it. That could be me, for instance, putting a bid into the market because I read a hot tip on HN. Or you may prefer to imagine an automated market-making model that moves its price for Ford because it just saw a big sell-off in GM. Or consider prices at the open each day, which are set before any transactions take place.