What do you think about this for a theory...
(Scroll wheel use is a big part of this theory by the way)
If you think about the way someone views a web page they're unlikely to give up after scrolling through 70% of it, they are more likely to speed up, scanning faster and faster and skipping bigger and bigger blocks of content until they arrive SLAM at the bottom of the page. They keep scrolling with their scroll wheel, but to no avail, they have reached the bottom. They have to focus on the screen to work out why it has stopped scrolling, and during this extra nanosecond you have their attention because they've got to decide what to do.
So this is where you stick a block of Google Adwords.