I am a little fuzzy but that is not exactly how it works.
It is a prediction based on advanced analytics that take into account every player, weather, stadium, etc.
It is meant to be accurate, but you are correct that they want it to be even and don't want to be leveraged to much on one side of the other and so when betting starts spread will adjust accordingly.
It is absolutely NOT a prediction by Vegas as to who will win. The initial line is created as a prediction on how they believe the avg bettor will bet. Most people perceive the Bengals as the stronger team and Vegas expects higher bets on them. Therefore, they set the line to get money moving our way. As the bets come in during the week, the line moves.
Again....Vegas wants EQUAL bets on the two teams. If they get this, they automatically win by earning the juice.