Google's head of webspam, Matt Cutts, put out a new Webmaster Help Video, responding to the user submitted question:
Under which circumstances will Google decide to display multiple results from the same website?
"The answer has changed over the years," he says. "But the high level answer is, when we think it's useful and it doesn't hurt diversity too much."
Cutts talks about a strategy Google used for years, called host crowding, where Google would group results from the same site together, but says people would get around this, and game the system by using different subdomains. He also talks about some other limitations of host crowding.
Discussing how things are these days, Cutts says, "You want to show as many results as you think is useful, and that's the tricky bit. What the user is looking for can vary depending on what they're searching for. For example, if they type in something like HP or IBM, probably a lot of pages or a lot of results from HP is a good answer. So several people have noted that it's possible to get more than two, more than four, lots of results from Hewlett Packard if you search for HP. But that's OK. The user has indicated that's their interest by doing that query."
He continues, "But in general, what we try to balance is this trade-off between a good diversity of results, because you don't know exactly what the user was looking for, so you want to give them a little bit of a sampling to say, 'OK, here's a bunch of different possible interpretations. Here's what you might be looking for.' And then we also want to absolutely give the results that we think match the query well, and sometimes that can be from multiple pages within the same site."
"So there's always a tension," says Cutts. "There's always a trade-off in trying to figure out what is the best set of search results to return. There's no objectively true or perfect way to do it. We've varied our scoring. We've varied our user interfaces. And if there's one thing you can count on, it will be that Google will continue to test out ideas. Google will continue to evolve how often we think it's appropriate to show how many results from how many sites in the search results."
Google, as you may know, makes changes to its algorithm every day. Each month, Google puts out a big list of recent changes. Here are the changes Google made in May. Those are just the actual changes. Google also runs 20,000 search experiments a year.