WebmasterWorld Member, beren, said:
For the past year or two, AltaVista.com and Alltheweb.com have produced the same results as Yahoo.com for a given search. Today, they don't. AltaVista.com and Alltheweb.com still show the same results as each other, but they are different from Yahoo.com. I tried several searches on terms I don't usually monitor. In each case, the results were different.
Could it be that Yahoo is using alltheweb.com and altavista.com as a test search site?
Some history for you...
Very early 2004, Yahoo! was shopping for search engines to buy, so they no longer have to be dependent on Google's search engine. They were expected to use Inktomi which an engine they bought earlier, but then they bought AllTheWeb and Alta Vista. They made changed to both engines and released their own flavor of Yahoo! Search.
Forum discussion at WebmasterWorld.