NEW YORK (AP) -- It's been a tough football season for sports books.

FanDuel and its key rival DraftKings have both showed investors that the house can indeed lose, or at least lose more than anticipated.

Flutter Entertainment, which operates FanDuel and dominates the sports betting industry with a market value of $45 billion, said the latest NFL season has been "the most customer-friendly" with the highest rate of favorites winning in 20 years. Altogether, the bigger payouts to bettors cut an estimated $390 million from the company's revenue from most of November to the end of December.

Rival DraftKings made similar comments in November about more payouts to bettors. The companies now face a busy weekend of playoffs for the NFL, which dominates viewership in the U.S. and sports betting.

The favorites for this weekend's playoff games are the Los Angeles Chargers, Baltimore Ravens, Minnesota Vikings, Philadelphia Eagles and the Buffalo Bills, according to BetMGM. The Chargers are the lone road team to be favored. The Vikings were originally scheduled to play the Rams in Los Angeles, but the game has been relocated to a neutral site due to the wildfires in the LA area.

"It's a business that has some short-term volatility to it, depending on the sports outcomes, but investors have grown increasingly aware that comes with the business," said Robert Fishman, senior analyst at equity research firm MoffettNathanson.

Commercial sports betting revenue in the U.S. surged 27% to just under $11.1 billion through October in 2024, compared with the same 10-month period in 2023, according to the American Gaming Association.

The organization had previously forecast that American adults would bet $35 billion with legal sports books throughout the National Football League's current season. That would make it the "most legally wagered-on season" in the seven seasons that the NFL has had legal betting. It doesn't have any projections for NFL playoffs specifically, but expects wagers on U.S. sports for all of January to exceed the $13.9 billion registered a year ago.

Flutter and DraftKings have been growing steadily in size and are becoming more firmly established in the viewing experience for sports fans. That's a sharp reversal from less than a decade ago, when sports betting was relegated to official sports books in Las Vegas and in more shadowy betting markets.

A 2018 decision from the U.S. Supreme Court struck down a federal law prohibiting sports gambling. Now, 38 states and Washington D.C. allow online sports books, according to the American Gaming Association. Both Flutter's FanDuel and DraftKings count the NFL among their biggest partnerships. Other partnerships include Major League Baseball.

"Sports betting continues to be increasingly adopted as legalization rolls out across the country and awareness continues to grow," Fishman said.

Partnerships and broader demand for growth have prompted sports books to invest more in creating a more comfortable integration of sports gambling within sports.

Sales and marketing make up the biggest expense for Flutter, growing 25% to just under $3.8 billion in 2023. The company has a large international footprint and revenue rose 24.6% to just under $11.8 billion in 2023.

DraftKings, which is more focused on the U.S. market, has also been increasing sales and marketing spending, though at a slower rate. It rose 1.2% to $1.2 billion in 2023, while revenue jumped 63.6% to just under $3.7 billion.

Sports leagues, and the companies that hold contracts for broadcast rights to the games, benefit from increased advertising from companies like DraftKings and Flutter. They can also potentially benefit from higher viewership as bettors may be more likely to watch the games if they have made wagers on them.