Okay so everybody says the 318d and the 320d are the same engine block Just with different tunings.

I've read that you can tune the 318D up to be a 320D as a result of this, However, on the Bluefin tuning site it says you can squeeze and extra 42BHP out of a 318D and an extra 44BHP out of the 320D.

So why is it that if the 318D has the same engine Block etc as the 320D you cannot squeeze and extra 86BHP out of it?