In a pick 3 with 10 horses each there are 1000 combinations.

Say there's a 100k pool and no take out for simplification.

So each combo would have on average $100 on it, given equal odds.

$100, not much is it? Take's very little to skew a payoff , especially lower. It's actually a minor miracle that the payoffs are as close as they are to the 'expected' results.