That sly grin your bookmaker has had during recent football seasons can be traced to an impressive run by NFL underdogs, who have been increasingly covering the spread and springing outright upsets during one of the worst stretches for betting favorites in the past 20 years:
Underdogs have covered the spread in more games than favorites in four consecutive regular seasons.
Underdogs pulled 99 outright upsets last year, winning nearly 37% of games during last year's 18-week regular season. The underdogs' winning percentage was tied for the highest in the past 15 seasons, and the 99 upsets were the most since 2006. (The first-ever Week 18 featured six upsets, but even without those, 2021 was one of only five regular seasons in the past 20 years to feature 90-plus outright upsets.)
2021 was the fourth straight season with at least one underdog of 15 points or more winning straight up. 2021 had two, both by the Jaguars. There had been no upsets that large in the previous 22 seasons, according to ESPN Stats & Information.
In 2017, favorites covered the spread in 54.4% of games, the second-highest mark in the past 20 years. Underdogs started getting hot the following season, which, conveniently for those wily bookies, coincided with a ruling by the U.S. Supreme Court that jump-started widespread legal sports betting in the country. Since that 2018 Supreme Court decision, NFL underdogs have covered the spread in 53.6% of regular-season games. Nevada sportsbooks have set all-time records for amount won on football in three of the past four years.
"I think I can speak for most bookmakers ... when underdogs win, that's usually good for the house," said Adam Pullen, a veteran Las Vegas bookmaker with Caesars Sportsbook. "The sharp bettors are a different animal, but the public just loves to bet the favorites."
Pullen believes the amount typically wagered on underdogs is increasing, but he still estimates there is more money on the favorites in 85% to 90% of NFL games.
Bookmakers pay to attention weekly to the liability that builds up on money-line parlays, which have become increasingly popular in recent years and often are loaded with heavy favorites.
"We've seen underdogs winning more games straight up than they have in recent years, so that definitely helps when it comes to money-line parlays and teasers," Pullen said. "Usually when you just get just a few underdogs to come in, that's almost going to make your week."
NFL odds & ends: By the numbers
$20,000: The largest Super Bowl bet by amount risked that Caesars Sportsbook accepted during the offseason was $20,000 on the Bills at 13-2 with a potential net win of $130,000.
Other teams that attracted five-figure Super Bowl wagers this offseason at Caesars Sportsbook include:
$15,000 on the Broncos (16-1) to win a net $240,000
$12,000 on the Ravens (22-1) to win a net $264,000
$11,000 on the Colts (25-1) to win a net $275,000
$11,000 on the Packers (12-1) to win a net $132,000
$10,000 on the Saints (50-1) to win a net $500,000
$10,000 on the Buccaneers (40-1) to win a net $400,000
18%: The Bills, the consensus favorites, have attracted 18% of the total money wagered on DraftKings' odds to win the Super Bowl, more than double any other team.
$1 million: One of the very first bets Caesars Sportsbook took when it posted its odds to win Super Bowl LVII in January was $5,000 on the Jets at 200-1. The bet would pay a net $1 million if the Jets win it all.
$2 million: Multiple sportsbooks reported taking big bets on the Dolphins from a bettor in Tennessee on Wednesday. According to BetMGM, a bettor put $900,000 on Miami to win more than 7.5 games at -250 odds, $100,000 on the Fins to make the playoffs at +135 and $10,000 on them to win the Super Bowl at 40-1. The bets would pay a net $2 million.
On the same day, Caesars Sportsbook reported taking a total of $5,000 on two bets on Dolphins quarterback Tua Tagovailoa to win MVP at 75-1 and 60-1. Combined, the bets would pay a net $345,000. Entering the season, more money has been bet on Tagovailoa to win MVP than Patrick Mahomes or Aaron Rodgers.
700 yards: The over/under on Tagovailoa's passing yards for the season opened at 3,050.5 and grew to 3,750.5 by September at Caesars Sportsbook.
14.6%: The percentage of games that finished with a margin of victory of three over the past five seasons, the most of any number.
Most common margins of victory over past five seasons by percentage of games:
53.5%: The percentage of regular-season games that stayed under the total last season, a four-year high. An average of 46.0 points per game were scored last season.
49.6: The average points scored in regular-season games played in January, the highest of any month.
1: The number of seasons that home teams have covered the spread in more games than road teams in the past five years. Road teams have covered the spread in more games than home teams in the past four seasons, but it's close (51.7%).
880.5 yards: More money was bet on over 880.5 receiving yards for the Rams' Allen Robinson II than any other season-long player prop at PointsBet.
98.4%: At sportsbook operator BetRivers, 98.4% of the money bet on the Week 1 game between the Seahawks and Broncos is on favored Denver. The Broncos are 6-point favorites.
97%: In BetMGM's "Yes/No" odds to make the playoffs, the Lions are the most popular bet with 97% of the money wagered on the "Yes." Detroit opened at +500 to make the playoffs. The Lions enter the season at +350 to reach the playoffs.
23%: Lions coach Dan Campbell has attracted 23% of the total money bet on DraftKings' odds to win Coach of the Year, the most of any coach. Campbell is 18-1.
485.5 yards: The over/under on 49ers quarterback Trey Lance's rushing yards opened at 485.5 yards at Caesars Sportsbook and attracted more money than any other preseason player prop. The number had climbed to 625.5 yards by September. More money was bet on the under on Lance's rushing yards than any other player prop side. The over on Lance's rushing yards has attracted the third-most handle out of Caesars' player props.
$500: A bettor with PointsBet risked $500 on Jaguars quarterback Trevor Lawrence to lead the league in passing yardage at 400-1.
*ESPN Stats & Information researcher Zack Rosenberg contributed to this article.