Please excuse me if this has already been noted,
After reading the post by themaninthejar I carried out a site search, but insted I put site:domain com KeyWords. I was astonished at the pages that were being listed for the keyword, most times they only were partialy related, the relationship being that they were from Same folder.
If this is the case on everyones domain there is sure to be a pattern that can be looked at.
I have already seen something of a pattern. The pattern I see is
1. I have external links to these pages,
2. They are all linked from my home.
3. Other pages I have external links to but are more than 1 click in appear to be unaffected, and appear #1 on sitesearch Keyword.
There is a definate pattern here, either it is because they are linked from my home page or because they have external links to them and linked from my home page. Try it and see if you find the same.