With fixed odds betting, the odds shown when the bet is accepted are the odds paid if it wins, even if the price later moves. This differs from pool or parimutuel betting, where the payout depends on how much is bet in total and is only known after betting closes. Fixed odds include a margin for the bookmaker. A bettor who takes a price that later shortens has, in effect, gotten a better price than the market now offers.
Example
A bet placed at 3.00 on a 10 unit stake pays 30 if it wins, even if the price drops to 2.20 before the event starts.