Different sportsbooks have different risk profiles, different lean tendencies, and different times when they sharpen lines. A book that's heavy on one side will adjust its price; a book that hasn't taken much action might still be at the open. When the books disagree by enough, the combined implied probabilities of both sides drop below 100% — meaning a bettor staking both sides locks in profit no matter the outcome.
The catch: bookmakers actively monitor for arbitrage activity. Patterns like betting both sides of a market, betting the maximum stake on a soft line, or having an account with consistently positive CLV will get your account limited or banned.
Arbing is real, profitable, and legal — but it's a fast-burn strategy. Most arbers cycle through accounts as they get limited, and the better-known operators (DraftKings, FanDuel) limit faster than smaller offshore books.