I think it shows people who logged on most recently, and maybe you re-log on as you view the main page, but it is definitely to do with loading the main page, I always see myself

It shows the last 5 people logged on. Sometimes you refresh and another users logs on which puts you in the 2nd from the left.

you could see the same list (if no one else clicked anything) by going to the user profiles list. You are always on the top of that too.

It's the "you've just updated the site" thing. The people before you (if any) are the people who connected between your request and your page being delivered.

Yeh the same happens to me, at first when I started going to this website I thought "hey I must be pretty popular!", but then I realized I was a newb with like 50 post so I kinda ruled that out...