I'm curious as to what their test protocol is that it has dispelled every other posting on this topic. A person told to buy shoes at Amazon.com isn't going to be stifled by a scrollbar, but someone who isn't being directly told to interact with the website in a given manner doesn't seem as likely to scroll down looking for content that isn't found on a sparsely populated top page.

(Related: http://www.useit.com/alertbox/word-matching.html)