Key Numbers and Off Standard Lines
From Bodog Sportsbook

Football is unique among the major pro sports in terms of how points are scored. In baseball, hockey and soccer, all scores are valued as a single point. These sports also tend to have low scores and the average margin of victory is extremely low so point spreads aren't used in the same way as they are in football and basketball, and moneylines are the most common form of betting. Basketball scores come in increments of 1, 2 or 3, but the high amount of scoring makes for an even distribution of final scores. As a result, basketball is perhaps the simplest sport to wager on in terms of lines offered.

Football gets complicated because the scoring comes in chunks of 3 points (for a field goal) and 7 points (for a converted touchdown). There is also a small amount of other scoring possibilities with safeties providing 2 points, missed extra points making a Touchdown worth only 6 and the two point conversion making 8 a possibility. Scoring in football tends to take place on average only about eight times a game so, unlike basketball, the final scores do group around certain numbers, known as key numbers. The major key numbers are 3 and 7, but 1, 4, 6, 10, 13 and 14 have a high likelihood of being the final margin of victory as well and are sometimes referred to as minor key numbers.

If we look at how all these numbers relate to 3 and 7, its easy to see why they occur frequently. The following chart shows the frequency of certain final scores for the 2001 NFL season as well as the relationship of the number to a combination of Touchdowns and Field Goals:
 

There are two ways a sportsbook can avoid the risk of key numbers. The first way is to know what the closing line will be and to get to it as soon as possible. The other way to balance action is to alter the odds associated with the pointspread. Most pointspreads are offered at standard odds of -110, meaning you must risk $110 to win $100. (Remember that when sportsbooks move a pointspread, the odds donít change, as the spreads only affects whether your wager is a winner or not. Changing the odds, on the other hand, doesn't affect whether your wager is a winner or not, but instead affects the payout.) By changing the odds away from the standard of -110, the House can make the same line more or less attractive to people looking to place wagers.