You are correct that the bookies can predict the expected outcome of any match with a great amount of accuracy. The main difference is that the average punter is not always rational, so the bookies will adjust their book to where the demand is coming from rather than the actual chances of a team winning the match. That often means there is value in going against the crowd if the odds are an unfair reflection of their actual chance of winning. For example, Man U went on a good run when Solskjaer was originally appointed. The stats for their games showed they were far overachieving based on their actual chances. There was great value in laying Man U and as most people are aware, the wheels soon feel off that bandwagon.