Transporter chief is
an engineering position.
I would assume that "lower ranking" personnel is always specialized in some way.
Just like a starship's Science department consists of astronomers, anthropologists, exo-biologists and so on, the Engineering crew has transporter technicians, warp field specialists etc. Only the department heads and maybe their deputies deal with all scientific or engineering matters in a general way. On the Enterprise-D, O'Brien he just wasn't serving as a department head, while on DS9 he was.