Please read the site stats entry here:
These are the terms, words, and phrases people use, through various search engines (i.e. Google, Yahoo, Bing), to find posts and pages on your WordPress.com blog. Please note that these do not include the terms which your readers use within your blog’s Search Widget or any other search form on your blog.
Some search engines don’t reveal search terms for privacy reasons. That’s why we don’t always know which search terms were used by visitors who arrived at your site from a search engine. When we don’t know the search terms, we show them as “Unknown search terms”. http://en.support.wordpress.com/stats/#search-engine-terms