What do the indented secondary pages in Google results signify?

[Jon_King]

Many times there are two listings for a site that is returned in a Google serp. What is the reasoning behind the two-page return for a single site, I cannot seem to find a common theme?

[Mohamed_E]

Assume that one of the pages ranked #3 and the other one #8. Google combines them, showing the searcher that this site has two more or lesss equally relevant pages.

The criterion for combining them seems to be that they would rank on the same page. Thus, with the normal 10 results per page, if the two pages were $3 and #15 they would not be clustered. But if you displayed 20 results per page they would be.