You’re right that the horizontals should, [i]on average[/i], pay more than the equivalent win parlay. This is because, even though the takeout on horizontal exotic bets is higher than on win bets, you only get hit with that takeout [u][i]once[/i][/u]. The more races involved in the bet, the greater this effect.

At Delmar, the math implies the bets should pay, on average:

DD: +8.1% more than the win parlay
Pk3: +26.2% more than the win parlay
Pk4: +49.2% more than the win parlay

In the results you posted:

$2DD paid $233.40. This was -10.5% less than the $260.76 win parlay. The expected payout was $260.76 x 1.081 = $282.
$1Pk3 paid $4,543.10. This was +92.5% more than the $2,359.88 win parlay. The expected payout was $2,359.88 x 1.262 = $2,978.
$.50Pk4 paid $7,900.05. This was -19.3% less than the $9,793.49 parlay. The expected payout was $9,793.49 x 1.492 = $14,612.

As Boscar noted, results can vary widely between any individual sequence, but over the long run they do converge on the expected payout based on the math.

A couple of years ago I looked at the results for the winter/spring meet at Santa Anita to confirm the math. Even though my sample size was only 80 days, I found that the results were already converging to the expected averages:

[u][b]2012 Santa Anita Winter/Spring Meet (Races 1-5 early Pick 5 sequence - 80 days)[/b][/u]

Median DD payout: +12.9% more than the win parlay vs. +8.1% expected
Median Pk3 payout: +32.2% more than the win parlay vs. +26.2% expected
Median Pk4 payout: +61.2% more than the win parlay vs. +49.2% expected

Two thirds of the DD payouts fell between -13% and +42% of the win parlay vs. the +8.1% expected average
Two thirds of the Pk3 payouts fell between +7% and +65% of the win parlay vs. the +26.2% expected average
Two thirds of the Pk4 payouts fell between +11% and +110% of the win parlay vs. the +49.2% expected average

This should give you an idea of expected payoff for each type of horizontal wager and the variation you are likely to see around those expected payoffs.

- Rocky