One reason is because the sports books run on a very small profit margin, 5%, which means 95% of the money gets returned to the bettor. They have to be very accurate on how they set odds and watch the flow of money, because the odds would change if the money changes and so on. It's a continual monitoring of behaviour, because sports books only offer bets on sports that are widely known and widely accessible, so there is a knowledge base that everybody has access to in a lot of ways to make those bets. It is something that is very keenly watched, because if they don't get it right or money moves the wrong way, sports books can lose money, and that's happened in Nevada and Super Bowl games in the past. It does happen where the overall book would lose money on a game because they either didn't get it right or the money went the other way on the odds. So they are keenly aware of where money flows.
On February 16th, 2012. See this statement in context.