From Search Engine Land:
Google announced on the Search Liaison Twitter account just now that it has updated its search results to show a more diverse set of search results. That means Google will aim to show no more than two results from the same domain for a particular query in the top results.
More diverse Google results. Searchers, along with SEOs, have complained over the years that sometimes Google shows too many listings for the top search results from the same domain name. So if you do a search for a particular query, you may see 4 or 5 of the top ten results from the same domain name. Google is looking to not show more than two results from the same domain with this search update.
From 9 to 5 Google
There are exceptions to this policy when Google determines it’s “especially relevant to [return multiple results from the same site] for a particular search.” For example, specifying a specific retailer in your lookup. Subdomains are considered part of the room domain, but there are again exceptions when applicable.