How Yahoo creates descriptions for your site

Yahoo seems to use only the first part of the meta description which is complemented by a text snippet from the searched page that contains the searched keyword.

If a web page doesn't have a meta description, Yahoo will use the description of the web page from Yahoo's directory (if the page is listed there).

If a web page has no meta description and is not listed in the Yahoo directory, then Yahoo will display sentences from the found web page that contain the searched keywords.

source- http://www.free-seo-news.com

Yeah! Yahoo is Smart.