When searching on Google, it's not uncommon that if you drill into the listings, Google will come back with the response "In order to show you the most relevant results, we have omitted some entries very similar..."
What happens is that in these cases, Google is only showing you the documents it thinks are substantially unique for that search. So if there are a lot of duplicate, mirror or near-mirror pages, these are only listed if you choose to try the search again using the "If you like, you can repeat the search with the omitted results included" link that Google provides when this duplication roll-up happens.
But c'mon -- out of 278 million matches for a search on Yahoo, only 38 unique documents are found? That's what our forum members are discussing over here: Only 31 results shown for Yahoo. I say 38 because when I searched for yahoo today, that's how far I got before getting the "pages omitted" message.
Now add the command &filter to the query like this: http://www.google.com/search?q=yahoo&filter=0. Suddenly, the duplicates no longer seem so duplicate.
So wondering minds wonder: filter, bug or feature. I'm checking.
Postscript: See the forum link above, where Google has posted information saying that it's related to the result crowding feature they do.