Why the special treatment?

Now I'm granted a bunch of links

[Brian]

In my five years or so of operating my site, I've wondered why some sites with homepages topping the serps get a mini-index under their entry pointing to different parts of the site [eg microsoft has them to its downloads page etc].

Well, today, I got my own. Somehow Google has actually picked the top two most important subindex pages, and also my sitemap. Spooky...

Anybody know what the criteria are? Is it PR or something (my green bar says 5)?

Happy holidays.