The pari-mutuel pool
Wagers of the same type are combined into a pool. After the applicable takeout, the remaining pool is distributed among winning tickets.
See also: What a minus pool is · How carryovers work
Why odds can change
As more money enters the pool, each horse's share changes. The odds displayed before the start can therefore differ from the final price used to calculate payouts.
Reading fractional odds
Odds such as 3-1 express potential profit relative to the stake before rounding and pool effects. They are not a probability guarantee and are not the same as a final payout quote.
See also: Odds payout chart · What morning line odds mean
Price is not prediction
Lower odds generally reflect more money wagered on a horse, not certainty. Long shots win and favorites lose; decide your limit before the race rather than reacting to a changing board.
Further reading