That's basically true for horse racing because all the relevant information is known in the minutes before the race and you can make your final bet. For a prediction market you would actually expect that if you have an informational advantage, the odds will move closer to your estimate over time as that information is revealed or processed by other market participants. The time frame on Bets of Bitcoin is long enough that genuine information will be revealed in the meantime for many bets. Because later bets aren't "worth" as much as earlier bets, though, it's impossible to say how much this would affect the actually payoffs without having a specific example to consider, though.