Google doesn't pretend to have every font you might want available in Google Web Fonts. Now, they're pointing you to alternative sources when they don't have what you're looking for.
In a brief post the the Web Font Blog, Google software engineer writes:
We know that finding the right font for your website or blog is a personal choice, and there are many great fonts available to choose from on the web. Now when you search for a font that isn’t available on Google Web Fonts, we show you additional fonts available from Monotype. Each result is shown in the actual font so you can easily preview your options. To get more information on a font, simply click the link under it’s name.
Google says it is working to add results from more web font providers in the future.