Its more to do with the fact that the offense takes longer to gel than the defense

There has been a lot of change on offense, no lance moore, no sproles, no graham yet
it means that brees has to get his timing down with these new guys and also get used to how they run routes, because the player might be faster and brees needs to account for that

with the defense there are some concepts and schemes that guys have to learn, but it doesnt take as near as long as the offense does