The principle, I believe, is that the government has the choice of disclosing or not disclosing the documents. Even if it is a Cabinet confidence, the government can decide that it will or will not disclose documents to Parliament. In this case, the government elected to disclose some documents, but not others. I imagine the rationale is the same—namely that they are Cabinet confidences. That is left to the government's discretion.
On March 16th, 2011. See this statement in context.