Why does evince look different from all my other apps?

I have always understood that it is your window manager that determines how windows are decorated. I use fluxbox, and applications normally appear within a typical fluxbox window frame. But evince looks quite different and behaves differently too. For some time, I couldn't even work out how to move the window around; you have to grab it well below the top edge, otherwise it just gets taller while remaining in the same position.

I believe evince is a gnome application but I don't seem to have any gnome libraries so where is this weird window coming from?