Why does the first result in some Google searches return a listing with 4 subpages included, like this example?

Google views these sites as being an authority on the search query the user entered. As part of Google providing the best user experience it lets the user drill down into the site right from the SERP to get the content they want quickly and easily.

Spot on Graeme!

I have a couple of those for RE related KW in my market. (It's gratifying...and traffic producing) (grin)

Best;

Eric

__________________
Eric Blackwell
Technologist, SEO Specialist and Consultant

Louisville Real Estate My Home Real Estate Site

Search Engine Marketing, Coaching & Custom WP Blogs Learn how to get the MOST out of your already awesome REW site.

Real Estate Industry Watch - Real Estate Industry News Blog Get up to date info on the Real Estate Industry

What factors does Google use to determine the authoritativeness of the site, and how does it choose which pages to include in the drill-down?

The regular algorithm determines whether or not you are an authority on that search query. This involves inbound links, anchor text etc, etc. And of course ranking #1 for that query.

As far as the link list algorithm, there are assumptions but it is all in the algo black box. I think it is based on the number of times users click that link in regular SERPS in relation to the query and of course the big G's algo relevancy score of the destination page.

here's google's own response

If you are into google patents you can dig up some stuff on how they do it.