There are hundreds of search engines out there that are powered by either Google, Yahoo, Ask Jeeves or an other engine. Should you expect that the search engines that are driven off someone else's technology, provide the same exact results? Bruce Clay has always had an outstanding Search Engine Relationship Chart so you know who is powering who (it changes all too often). But even though Netscape is powered by Google, it doesn't mean you will see the same 10 results in Netscape that you will in Google. I believe this applies to Yahoo! powering AltaVista, Inktomi, AllTheWeb and so on. Even though Yahoo! Search is a technology comprised of the above three, they each are slightly flavored for their target searcher.
Forum discussion at WebmasterWorld.