When you really start to get into how sports betting works, you will run across something called line moves, and you need to understand why the odds would change. There are any number of reasons that the odds could change for a matchup, and here are just a few of them, although the first two reasons are usually the most common for sports bettors.
Something Happened To The Matchup
Sports are one of the most unpredictable events going, so the number of reasons why the odds would change are plentiful. For example, a team's star player gets hurt preparing for a game will definitely change the odds. If New England is favored by a touchdown over a team, but Tom Brady hurts his knee and is out, those odds would definitely move in the direction of the other team. Sticking with football, there is also weather if the teams are playing outside. At the beginning of the week, there could be clear skies, but as the game approaches, snow could be in the forecast and that could move the lines one way or another (possibly towards the team with the better defense). This is why you have to keep an eye on the news leading up to the game, so you can figure out if the line will change, and why. This is also not just for football; it is for all sports.
The Sportsbook Needs To Balance
However, the line move could simply be due to the sportsbook needing to balance the money coming in. A sportsbook doesn't want their players to only bet on one team, so they'll spend time making the line enticing enough to players on both sides, which maximizes their profit and the reason why they're in this game in the first place. For example, say that New England is playing Miami at home. The Patriots are always going to be favored, all things being equal (both teams are both healthy, etc.), but the sportsbook is going to try to come up with a line that players think the Dolphins could cover. The could start out with a line of -7.5, but keep moving up to -8 or -8.5 until they start getting money in on the Dolphins to even things out.
The Sportsbook Actually Believes This Line
Sometimes, the linesmakers just think that an outcome is going to happen, so the line is what it is. Most sportsbooks don't do this because they try not to take one side or the other; their main side is money and profit. But sometimes, an outcome is so plain to see that there is nothing they can do. If they were to do this, it would be late in the game when it is apparent that most of the money is going to come in one side regardless, so they might as well take a position. Again, this is very rare, but it does happen from time to time, so be prepared for this.