Google's search results are in a perpetual state of flux - as if you didn't know. As its algorithms change behind the scenes, and its index gets updated and refreshed periodically, the SEO community begins to notice broader trends which reveal how the search engine (at least somewhat) returns pages and why. And that's something that's important to know about.
Last week Google's Matt Cutts posted a video regarding a new change that was released related directly to the "diversity" of listings on the search results. Cutts (in the video below) indicated that the change was "coming soon" but with Google's recent Penguin 2.0 update, that change has finally seemed to emerge. The change relates to what's called "domain clustering" or when several listings are seen in succession (or in a grouping) on the search results from the same domain name.
Most SEO's don't consider this (domain clustering) too big a deal as many users (upwards of ninety percent) don't visit the second page - where most of those clustered domain results tend to appear (at least recently). The history of domain clustering is an interesting one (see below) though and shows the progress Google has made in balancing out the search results pages to achieve diversity in its returned listings. That history can also provide some insights into some common and popular SEO practices that you either might want to engage in or avoid outright.
Long ago, it was very common to see many results from the same domain. Google then added host clustering which prevented users from seeing more than two results per domain name on the results page. At that point, SEO's started using subdomains with greater regularity. Then, Google changed their approach again and expanded the clustering to a max of three or four results per domain. Improving on its approach again, Google then changed to show more diversity on the first page but less on the secondary pages. Essentially at this point, users would not see more than two result from the same domain on the first page but would often see several listings from the same domain on secondary pages. With this most recent change, users will see even less clustering on secondary pages - and that provides a better opportunity for websites looking to improve their rank.