Emperor will set seedless fruit under the right conditions. I have been told that it relates to cross pollination. We had two Emperor trees separate from the rest of the grove that set all seedless fruit one year. Shortly after that they got flooded and died. So, the experiment couldn't continue. The ones that get good cross pollination up in the grove set very few seedless fruit. We also had a Brewster that had 100% chicken tongue seeds one year. We thought that we had something special, but the next year it put out normal fruit. Based on those observations, I am going to say that the fruits with a large seed got cross pollinated.