I have a question about facebook friends?

[Megan]

I was checking out my friend's facebook page and noticed that she had the top friends application. However, I noticed that some of her top friends were not apart of her facebook friends. How is this possible?
Are these friends she used to have and then deleted?

[me]

Top friends is an application. These people might have set up their privacy setting as to only appear to their friends. The top friend application takes their pics and names and put it in the box.