Those are more questions for the people doing the research, but I would assume that there is a higher incidence of lost embryo's with overo crosses, combined with the very low incidence of live lethal white foals being born, as opposed to the genetic cross 25% chance.
The evidence I'm refering to are the dead white babies, yes. Who said horse people were ethical? Whatever gave you that idea? (tongue in cheek) HYPP, HERDA, LW, etc... those should give the hint that people rather breed for money and prestiege than maybe what we SHOULD be breeding for. People tend to hide things that disgrace their stallions. HERDA horses were said to have "Cutter's disease" before it was named HERDA, because the affected horses were living happily until their second year, when they were saddled. Next day, skin fell off, horse just disapeared out of the barn and the trainer denies ever knowing about that animal. It was buried out back, because they didn't want anyone to know. And now we have another genetic disease to worry about. Whites are similar, and overo's are popular, so I have no doubt they were treated the same way. I'm not saying everyone does that, but underreporting isn't uncommon. Luckily people are becoming more educated and more ethical, and diseases like HYPP and testing for LW is becoming more common and more unaccepted, hopefully improving the breed and helping to eliminate those diseases.