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.

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' ;)