I think the figure you refer to is the "mark up" that bookmakers use to balance their books. I will attempt to show you how this works by means of an example. I am sure someone more qualified will give you a more in-depth answer.
Example: In a three-horse race, each horse is reckoned to have an equal chance of winning ( 2 to 1). The bookmaker, in order to make a profit, would adjust the prices, so that each horse was, say, 7 to 4 (1.75 to 1).
Whichever horse wins, the bookie would pay out �2.75 to every �3.00 that had been staked, giving him a profit of 25p in every �3, or 8.33p in every pound.
That would make the percentage of this particular race 108% (100% being the break even figure). It obviously gets more convaluted when there are more participants. In big races (i.e. the Grand National) it is not out of the ordinary to see a percentage figure of 130% or higher. Thats why you never see a poor bookie.
Hope this helps.