Generally, Google tries to show a varied set of search results, which among other things, means they’ll only show up to two results from any single domain for a query (at least, on the first page). Exceptions have always existed. For instance, if you search for a domain, Google might only show results from that domain (see a search for [Google] for an example of this).
A few months ago, they started showing more than two results from a single domain for other types of branded searches. At the time, Google said that this change was based on searcher intent: “this happens for searches that indicate a strong user interest in a particular domain”. (You can get tips for taking best advantage of this change in a recent 100% Organic column.)
Now, Google has taken this idea a step further “so that, when appropriate, more queries show additional results from a domain.” They say that:
“As before, we still provide links to results from a variety of domains to ensure people find a diverse set of sources relevant to their searches. However, when our algorithms predict pages from a particular site are likely to be most relevant, it makes sense to provide additional direct links in our search results.”
Based on perceived searcher intent, Google may show up to four results from a single domain (and might do this for multiple domains) and some of those results might display single-line snippets. Google says this latter change is to keep results compact, but so far it appears that Google still lists ten total web results in these cases.
The results may also include the “more results” plus box, as you can see with a search for my site:
What does this mean for site owners? On the one hand, if you’re ranking near the bottom of the first page for a query, you might now be pushed to page two if a site above you expands to four results vs. two. On the other hand, in the cases where your site expands to four results, you have new opportunities to showcase the content you have to offer. Take a quick look at the queries that drive the most traffic to your site and check what pages are now showing up and what your shortened snippet looks like (obviously, I have some work to do on mine!).