The naming custom is the surname goes before the given name. In “Ensign Ro”, Picard mistakenly says Ensign Laren. He is corrected, being told that her surname is Ro and some Bajorans try to fit in, reversing the structure.

I would think that Seven would be more comfortable within the crew dynamic of Voyager, addressing her by their surname.

EDIT: Being an xB, I would imagine Seven would already know this from Unimatrix Zero assimilating some Bajorans.