#1 Result extra links

[tomfallenstein]

I have a question.

How come sometimes the #1 result has extra links, showing its subpages? Example would be search for "star wars" or "halloween costumes"

Goolge shows some subpages of the top results. I can kind of understand this for star wars since it is obviously THE authorative site for that search, but HalloweenMart is not the authorative site. In fact Buy Costumes is the number 1 halloween store, so how does google choose which sites show that?

Any ideas?