As mentioned before, there are hundreds of potential variables that can factor into an initial betting line, and the same number applies to the rationale behind a shift in the odds. Game lines can also shift simply due to the amount of money being wagered on one side of the spread or odds. For instance, if a book gives the 49ers a 4.0 point spread advantage and an excessive amount of action comes in on San Francisco covering, the margin will shift upward until the bets equal the volume of their opponent.