How are basic search results generated?

Basic Solr searches default to an OR operator, meaning there is an implied OR between each of your keywords. This is why a search for John McGee will return more results than John AND McGee, which in turn has more results than "John McGee" in quotes.

However, unless you have changed the way search results are sorted (by score), the full results of an AND search will show up first in an OR search, before any results that do not contain all keywords are listed. So while an OR search turns up more results, the initial set of results are no different from doing an AND search.

Solr does this to add serendipity to searching -- including possibly-related results that do not match all keywords, to provide more paths a researcher may choose to follow.

At present there is no way to change the default operator for basic searches. Users may limit their searches by using Advanced Search, which you can encourage by adding an Advanced Search block to your search results page. You can also limit the number of results that come back by using Defined Query Fields instead of letting Solr determine relevancy.