Maybe it's not so much of a mystery why they were able to come up with horizontal payoffs. Those payoffs don't seem to change that much, if at all, and are available long before the race is run in the will-pays whereas the WPS, Exacta, Tri, Super are constantly changing before a race is actually run. I could be wrong about that as I don't often bet the Daily Doubles, P3s,P4s,P5s, and P6s.
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