"and" vs. "&"

While I have loved the film Adaptation really even before I saw it, I just noticed something about it. It has the credit "Screenplay by Charlie Kaufman and Donald Kaufman." According to the Writers Guild of America regulations, "&" is used to join two writers who work together on the same draft of a screenplay whereas "and" is used to join writers who work on separate drafts. Is this significant? Is this to imply that there was a draft by Charlie that the studio didn't like, and so they took it to Donald to rewrite it?