My theory would be that Google wants a range of different answers within the top 10 for any given search term, to give it the best chance of satisfying that search when it isn't quite sure of user intent. So it's not going to put 10 versions of the same answer in the top 10 in that case. It will maybe try to have some sites in there that provide some variety. That would be why two sites that are carbon copies can never rank both together. Instead, some other site will get in there which doesn't compete with the other results on authority, but does provide a variation on the naswer to the search query. That would be why sometimes you will look at the top 10 results and wonder how the hell that one site ranks in there when it's got like, 20 RDs and nowhere near as many articles as the others, or the articles are all less than 500 words, or whatever. You think it shouldn't rank, but it does because it's just different enough.