From what I can tell, a site with a logical hierarchy (file structure) is more likely to receive site links than a site which doesn't. The subpages seem to be an attempt to determine what the important sections coming from the homepage are. And obviously, they're trying to look at #1 results which seem to have a clearly navigational intent to them. (Lots of sites have sitelinks if you search for them by their name, for example, because the thinking behind the sitelinks seems to be that sitelinks would be useful to someone looking for a site by name.)
|