Maybe it's about the search intention. I recently heard something like that: When google understands/believs, that the user want's to buy something, you can only rank with a shop site (category or product site for example). If google understands, that 99% don't want to buy anything, then you wan't rank with a shop site. With shop site I don't mean the domain, but the art of the site (cart buttons & stuff).
I really don't know, if this is true. It's surely not as strict, as this guy told me. But it's sounds somehow logical to me.