So who wrote 'The Hulk'?

An interview with Ed Norton presented in anticipation of the film's release on DVD was posted on Movies.ie recently. The interview is generally very interesting as Norton discusses the film, his role, his love of comics, and the issues inherent with portraying an iconic pop-culture character. Of particular interest to anyone following the 'screenwriting issue' with the film is this specific Q&A:

Q: Is it true you wrote the screenplay for the movie too?
A: "Yeah, I came in to this and wrote the screenplay."

Interviews are often edited so Norton may have offered more than just this one sentence on the topic, but it is not the first time Norton has been quoted as saying he wrote the film. If Norton is to be taken at his word, why did Zak Penn get full screenwriting credit?

Who wrote 'The Hulk'?
---
FURTHER READING: /FILM AUGUST 15, 2007
FURTHER READING: EW.COM APRIL 16, 2008
FURTHER READING: MEDIAPUNDIT.NET JUNE 4, 2008
FURTHER READING: EW.COM JULY 25, 2008