One of the most common questions I see from searchers and SEOs is why the total results figure on Google's search results is wrong or off. If you do a search at Google, Google will show you "about x results" at the top. Here is a picture:
This figure should not be trusted.
Danny Sullivan recently posted a comprehensive article on this named Why Google Can't Count Results Properly. In there, he highlights a comment from Google's Matt Cutts where he explains why less may mean more. Let me quote:
As to why the query [A B -C] can return more estimated results than [A B], that’s easy to explain. The query [A B -C] causes us to go deeper through our posting lists looking for matches, which can lead to more accurate (and larger) results estimates. Other things can cause us to go deeper in finding matches, such as clicking deeper in search results. Results estimates can also vary based on which data centers or indices your query hits, as well as what language you’re searching in.
I have never seen this explanation before, but now it is becoming popular.
I spotted a Google Web Search Help thread where a different Googler used a similar explanation.
Gideon Wald a product manager at Google Search said:
I'm not at liberty to discuss many aspects of Google's search algorithm (nor am I personally familiar with most of them!), but I can say that a common source of this type of discrepancy is that some kinds of searches cause us to look further within our index than other types of searches. Typically, this tends to happen when the user issues a query that imposes some sort of specificity or structure over and above simple keyword matching (e.g., using the "-" operator to explicitly exclude words). In such cases, we do a deeper scan of our index, since these queries, being more complex, often have fewer results. I can't say for certain, but I might guess that quoting your search terms is one choice that triggers a deeper search.
The more specific or detailed the query, the deeper Google will reach and thus it might show more.
But I am not sure how this answers the site command issue. I basic site command is a pretty deep natured query, but it isn't all that accurate. I guess adding keywords to the site command always can provide a deeper search. But both are fairly deep in nature.
Forum discussion at Google Web Search Help.