It's statistical information arbitrage. Let's say you go to the futures market: Seller pays a contract-buyer $1 if it rains tomorrow. How much will seller cha

It's statistical information arbitrage. Let's say you go to the futures market: Seller pays a contract-buyer $1 if it rains tomorrow. How much will seller charge for that contract? Bookmaker using forecaster A will sell that contract for $0.51 every day, and earn a profit of $0.01/contract, on average. Bookmaker using forecaster B and their clients will buy that contract on probably sunny says, and bankrupt Forecaster A. Similar result if Bookmaker B sells contracts to Bookmaker A, at $0.11 on probably-sunny days, and $0.90 on probably-rainy days.