I find a study like this to be highly suspect. It's a 28 page story, hardly anything special. Have them read an actual novel instead. In 28 pages the "plot" is going to either be very convoluted or extremely thin. I find it just as likely that the 28 page story bored the kindle folks half to death and they didn't bother trying to recall it.
You're going to have to do better than 28 pages. That's barely a chapter in the books I read.