That could be because we tend to put the most important stuff above the fold. They analyzed real websites that (we would think) would tend to do that. My gues

That could be because we tend to put the most important stuff above the fold. They analyzed real websites that (we would think) would tend to do that. My guess is: if you capture their curiosity, they will scroll. Otherwise, they probably won't. Simple, I know, but I'm going with it.