I would argue that there is something about the physical interaction that is scrolling. you can scroll at the same time you are reading--this makes the text more of a moving target, and then your "reading" becomes mere scanning. it's hard to shift out of that shallow level of engagement once your there because it's by definition a distracted state, you're trying to just get the gist of this and move onto the next thing you just know is out there which you will actually want to read.