When performing a search on the Codex, you’re presented with a slew of search results. However, not all of those results are within the Codex. The search portion of the Codex is powered by a Google custom search box which not only presents results from within the Codex, but from across WordPress.org as well, mainly the support forum.
While performing a search for Conditional Statements, the first result was the one I was looking for. However, if you want the results to strictly be within the Codex, I came across this link shared by Otto on the Documentation mailing list. While giving this method of searching the Codex a try, I found it difficult to find the Conditional Statements page I was looking for that was easily displayed by the Google Custom Search box. Even by checking each box, I failed at finding the page using both Conditional Statements and Conditional as my search terms.
My advice, stick to using the Search box that exists on the Codex page.
That’s because the Wiki built in search is very exact. It looks for the page name, then the words close together, and as you may have noticed, the page name is ‘Conditional TAGS’ not statements. And the term “Conditional Statements” isn’t anywhere on the page. You’d have to add it for the wiki search to get it.
Of course, the real answer is ‘Google’s better at search than anyone else’ ;)