The odds aren't just about how good the horses are, they are about making sure the bookies who set the odds make a profit or don't lose too much. If lots of people bet on a horse, they will shorten the odds so they don't have to pay out as much. That's true whether the horse places or wins.
When you place a bet on a horse you can either place the bet at what the odds are at the time you place the bet, or go for Starting Price (the price when the race starts). If the horse is good, the odds on it will shorten nearer the race, so if you think the horse will do well, it is worth making sure your bet is locked at the odds that were set when you placed the bet.