In the perfect world the line that the oddsmaker puts out late Sunday night is the same line that is still in play the following Sunday morning when the majority of the bets are made. The world, however, is not perfect. So lines will fluctuate during the course of the week, some times only by a half-point, and other times by two or three points. There are times when the actual favorite changes. Where one team might open as a 2-point favorite and by game time has become a 2-point underdog. For instance, in week 13 of the 2018 NFL season the Cincinnati Bengals opened as a 2-point favorite over the Denver Broncos. When word got out that the Cincinnati quarterback Andy Dalton would not play, the line changed and the Denver Broncos were a 3-point favorite. By the way, Denver won and covered.
What causes these changes can be two-fold. Remember, as we said earlier, the goal of the oddsmaker setting the line is to get equal amount of money wagered on both teams. So as an example if the Green Bay Packers are a 3-point favorite over the Minnesota Vikings and all of the early money is being bet on Green Bay, the oddsmakers will move the line to perhaps make Green Bay a 4-point favorite to entice the public to bet more on the Minnesota Vikings. It works the other way as well. In the same example if all of the early money is being placed on the Minnesota Vikings then the line could drop to Green Bay as a 2-point favorite to entice the bettors to wager on the Packers. Another factor in a line changing are injuries. If a key player, especially a quarterback, gets injured during the week at practice, or if word leaks out a player is going to miss the game because of injury that could move the line at times drastically.