This was quite deliberate. Kill Bill is a very committed homage to the B movies that came out of Hong Kong in the seventies and early eighties. Guess what sorts of watches the actors in seventies Hong Kong B movies generally wore? You've got it: fake Rolex. As a result of this, the early fans of these movies would go out of their way to get 'authentic' fakes - as worn by... If you check out the back pages of some of the early fan magazines they are crawling with 'bolex' adverts.
It's an ironic twist that would delight Tarantino; in this case, a fake Rolex is more authentic than a real one!