Compare Yahoo! search results in AltaVista and AllTheWeb and you may notice difference in results. In a WebmasterWorld thread a member wrote: "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?"