Most professional bettors will wait until bet limits increase and are able to get a sizeable bet down before placing their bet. As the bettors place their wagers, bookmakers gain information a more accurate number than the opening number is formed, and that number becomes widely available. In most sports, the opening line is posted the night before the game, sometimes referred to as the ‘overnight line,’ and books won’t increase their bet limits until the following day. Occasionally a sportsbook might post a number that is differing from the market, which tells you the bookmakers’ have their own opinion on where the game should be lined, or they’re trying to attract more bets on a specific side. Those books won’t have to come up with their own line, they simply copy the market’s current consensus number. Most sportsbooks won’t post their own line/number until they feel comfortable enough with the market to start taking their own wagers, as a consensus number forms. Most opening lines aren’t generated by “Vegas,” but are opened offshore, with only a few shops taking a risk by posting true opening numbers.
They use the opening number to help gain information from the betting market and come up with a better number. It doesn’t take much to move a line shortly after it opens, given bookmakers aren’t taking large bets yet.
