Frank told me that Sam at Pizza City uses So Strong flour. The slice my granddaughter ate was from the same pizza that was made at market by Frank. Frank had purchased the dough ball from Sam before he came to market. Do you think maybe more oil in Sam's dough might have made the crust tender when using the So Strong flour?
It could be the oil, and even the sugar together with the oil, but you would need a fair amount of them to have that effect.
It might also be the way the slices were protected until your granddaughter ate the slices. In his book, The Taste of Bread
, Prof. Calvel discusses what happens to baked bread as it cools. Interestingly, the water content of the baked crumb is about the same as the dough that was used to make the bread, within about 1%. But how you protect the baked bread will affect the rate at which the bread stales. Professor Calvel says that during cold or dry weather, the bread should be protected from air currents to avoid excessive drying. In humid weather, the bread should be kept in a well-ventilated area to keep the crust from becoming overly soft. It should be kept under temperate conditions, about 75 degrees F, to avoid shock and premature staling from excessive cooling. Prof. Calvel goes on to say that if these precautions are taken, many breads may be kept fresh and in excellent condition for between 15 and 24 hours. Round loaves can have a shelf life of 2-3 days. I realize that bread is not pizza, but maybe similar principles apply to pizza slices. Most breads never see the inside of a refrigerator, whereas leftover slices often do. You didn't indicate how the slices were preserved but if they were put in the refrigerator, that will no doubt affect the slices and how well they eat.