It seems yahoo is doing human review and editing to search results, They purposely make a site rank on top, This was pointed out by Danny sullivan on searchenginewatch.com in this thread, this is what he posted,
In short, for some queries, some sites that came up had a special code
that seemed to indicate they may be hardcoded to rank tops for that query. I've
highlighted that below:
That's the redirect code for Cars.com, listed tops at Yahoo in a query on cars. H=1 seems to indicate the site was
hardcoded to show up in response to this query. In contrast, H=0 means no
hardcoding appears to be involved. As for this hiring, I'm with others that
posted. This isn't some push to build human spam hunters. They've already got
those. It looks like they want to increase the quality of results, and perhaps
in particular for cases where those results may be in non-English languages.
Spam elimination is part of that, but not the entire part.
It is very interesting and a bit worrying for many SEOs, you can read the whole posting of danny sullivan here,