True odds represent the probability of an outcome, but when you bet with a bookmaker the odds don't just represent the likelihood of the result. They also reflect the bookmakers need to make profit and the amounts of money staked on the different possible outcomes.
For example, if a lot of people bet on a particular team, or horse, then the bookmaker stands to lose a lot of money if it wins. In this situation, bookmakers will lower the odds on that team or horse to discourage people from backing it. They may also raise the odds on the other teams or horses to motivate people to put money on those participants instead, to even out their book. This can mean the odds do not always represent an accurate view of the outcome of an event, and can be influenced by volume of bets or bookmaker liablities.