Joel you are only considering one possible explanation, I think thats the point.
In light of the consistent performances by the "house" horses on that day, year after year, would you not be a tad suspicious of results there.
I'm not talking down to you, and I'm sure you probably remember the data, but do you recall Almutawakel's World Cup win?
Gimme an explanation for that performance if you can. Because years later, I'm still scratching my head.