From Pontifications: “The Church does not know more than the Apostles knew”

I have a question—OK, maybe a Socratic question. How can John Henry Newman, the 19th-century Anglican convert to Catholicism who virtually invented the idea of "development of doctrine," assure his correspondent that the Church knows no more than the Apostles knew? After all, both the Orthodox and fundamentalist Protestants would agree that the Church knows no more than the Apostles knew, but for that very reason deny that there is any such thing as legitimate development of doctrine.

Perhaps another way to frame the question is: how can "development" of doctrine not be "addition" of doctrine to the faith once delivered to the saints? I know the chorus that says it cannot fail to be addition, and thus illegitimate; but I'm interested in seeing somebody actually engage Newman (and Vatican II) on this question instead of dismissing development out of hand.

To paraphrase the Pontificator: "Some Christians don't believe in development of doctrine. They just practice it."