According to some discussions that are going on WebmasterWorld, it has been noticed that even though a lot of webmasters are redirecting their non www to the www version, Yahoo! is still listing their non-www as far as the main results are concerned.
Let us have a look at some of the discussions:
www.example.com redirects to example.com and has done so for quite some time now. Yet when I do a search for a two word primary key phrase, Yahoo shows www.example.com. A search for my domain name returns example.com. I was at #1 for a few years but lost some ranking in August for this search phrase. I'm not sure if the ranking loss is related. – By ChicagoFan67
I looked, and can't find any backlinks at all to non-www but the homepage shows up both with and without for different searches. – Marcia.
But I HAVE seen Yahoo list the homepage of sites in the past without www when doing a site: search on the domain, so it isn't something brand new with them. In fact, they're doing it now with an indented listing for one of the *big* discount retail chains for a query – one with, one without- she further said.