As I see search terms in Search used with OR operator by default and I see no way to switch or force it to AND. For example I just queried [XTS mode] in the stackoverflow and all results were irrelevant (the moment before I read a post where both terms were used)
Google always used AND operator by default. As I recall from my experience the noticeable exception is phpBB that uses OR by default. I know that this can be a very long discussion, but the well-known search engines is a strong point on my side, aren't they?
EDIT: I just read about the feature that AND results should be at top. Not always actually, or with this approach (all shown, AND first), suggestion would be at least mark the sections, so to see there's no AND results at all.
I'd say that the search works strangely, two examples:
the query I posted [XTS Mode] didn't have XTS in the first result,
Query [Delphi Register] didn't have "register" in the first result, but googling [site:stackoverflow.com Delphi register] led to the article http://stackoverflow.com/questions/517944/calling-a-delphi-dll-from-c-produces-unexpected-results that had both words