My university has the policy that until you have passed your PhD you can't be formally employed as a postdoc, you appear to be to everyone in the department but you are only being paid as a research assistant. I know this because my officemate was in this position for a few months. He was staying on continuing the work he did in his PhD but for the 4 months between submitting and formally passing (after his viva, corrections and the admin were done) the was a research assistant and then got a ~£5k a year pay rise when he passed and his position was changed to a postdoc. Apparently this is common practice in some universities (usually for continuing students), but apparently some universities can't do this if the position is advertised as a "Research Fellow", it can only be done if the post is advertised as "Research Fellow/Research Assistant" for technical reasons I'm lead to believe