Parimutuel betting, not sure - but there is always a house fee - that's how the house makes money.
A traditional bookie would have at least 5 to 10 % built into his odds as well (like if you were betting on even odds, liek a coin toss, it would be bet $110 to win $100 for either side (so the bookie has a 5% hold if he can keep the betting balanced, no matter who wins)
Isn't that a little high?