Pari-mutuel odds are calculated in an automated manner, but sportsbooks use fixed odds. They set opening lines, and they adjust them in an effort to balance the action. They want to take the same amount of money on both sides of the bets.
To explain through the use of an example, the standard vigorish on a football point spread is 10 percent. You have to wager $110 to win $100 on either side. Let’s assume the Packers are playing the Cowboys, and you bet $110 on the Packers. Your friend bets the same amount on the Cowboys.
Before the game starts, the bookmaker is holding $220. Your Packers cover the spread, and you win $100, and you get your original $110 bet back. The sportsbook paid out $210, but they were holding $220, so they get to keep the $10 profit.
If your friend does not make the bet, they lose $100. Obviously, they are taking thousands of bets so the equation is complicated, but this is the basic principle.
A racetrack does not have to balance the action to get its share, and this is the difference between pari-mutuel wagering and fixed odds betting.