I think we have to compare Yahoo with similar sites. It is difficult to find such sites but just to give an example:
Consider web hosting business. Yahoo is quite interested in it. Its home page mentions it at least twice and a few related terms like domain, personal web site and the the link takes it to a web hosting PR8 page. No SE will mistake that page for anything but a web hosting page. However, in serps for that phrase, its competitors, a few with PR8 but many with just PR6-PR7 pages are able to do better. (Yahoo seems to appear on the 4th page.) Other factors like the anchor text are involved too but part of the reason could be that the average PR of backlinks to these sites are likely higher than the average PR of backlinks to Yahoo.