I remeber to have heard, that the 737NG is not certified as a new design, but certified by a supplemental type certificate. This has the advantage, that the certification specifications are not the latest available ones for systems that you don't change compared to the old design. As an example, the brake system is still following the specifications of the first certified 737 (so late 60's standard).
By using this practise, Boeing is saving a lot of money, as they do not have to go the full certification process for all the systems on the plane.
Don't flame me for this statement, as it is "second hand knowledge", but it makes well sense to me.