Bookmakers and Odds Compilers
Bookmakers take bets on sporting fixtures for which the outcomes are difficult to predict. The major bookmakers employ a team of odds compilers to set their prices.
The odds compilers will work to certain profit margins or over-rounds. They may make £10 for every £110 they take and set their over-round on that market at 110%. To build this into the odds they take their calculation of each runner's chances and shorten some or all of their odds. Bookmakers are often criticised by punters for making it too difficult for them.
Bookmakers also apply factors such as which selections are likely to receive the most support.