Say I was a professional gambler and I had really investigated the statistics and other parameters surrounding an event. Mythical United are playing Made-up FC and Made-up FC have been set at 1/1 or evens. However, I happen to know that Mythical United are not playing four of their strongest players due to a sickness virus in the club and will be at a severe disadvantage. If I bet £1,000,000 five hours before the game then that is when the odds will change. Because of that, everyone else will see the change of odds and might think "Oh, I'm getting some of that!" It's a crude example but the sizeable bets are generally placed around 10-15 minutes before the event and that is why the market appears to "correct" itself.