Honestly, I think you under-estimate the general population's ability to learn how to use a tool. I think being able to explain why the results are what they are takes another level of understanding. But I think - and, yes, this is an unsubstantiated claim based on personal bias and anecdote - that many people have developed an intuitive feel for how to craft a decent Google query. They may not be able to tell you why they say what they say, but they can still say it.

Personally, I think Google is a good tool. I feel my mental model of it is relatively simple, and from that, I can mostly get what I want. This is also a good time to plug a great article on tools: http://unqualified-reservations.blogspot.com/2009/07/wolfram...

edit: My explanation was not that Google understand your search to be "used cars." My explanation was based on the assumption that you were looking for a place, online, to buy used cars from. The keywords "online" and "leader," muddied that. The search for "used cars" yields many places to buy used cars from, and the top entries are probably the "leaders."