From http://google.com/support/bin/answer.py?answer=499&topic=352:

"We call that collapsing. When we realize that some of the results for your query are duplicates, we collapse the duplicates so that you can find the specific result you're looking for more easily. Collapsing the duplicates makes the estimated number of results, as well as the overall number of results pages, appear to decrease from the total initially displayed.

To expand your results, just navigate to the last search results page. At the bottom of the page, click on the link that reads, "In order to show you the most relevant results, we have omitted some entries very similar to the [number] already displayed. If you like, you can repeat the search with the omitted results included.""