I suspect that Yahoo's SERPs are not as refine.
Using "dog sweaters" as an example, Google gave me 417,000
http://www.google.com/search?hl=en&q...+sweater&gl=us
and Yahoo gave me 39,000,000
http://search.yahoo.com/search?p=dog...UTF-8&fp_ip=CA
So I think Yahoo is returning "dog" + "sweater" + "dog sweater" ?
Or anything mentioning dog + sweater
And that might include some webpage on a department store website with "dog carriers" and "men's sweaters" ?
My theory anyway. Which is why you probably want to stick with Google for estimating the competitive landscape.