I'm not advocating doing no gameplanning, to be clear. I went through every play of every game of mnmyers's St. O's season in planning for that NC game. So, it may be a question of defensive philosophy as much as anything else.
From an offensive standpoint, I can manipulate the variables easily enough to show a wide variety of looks, both in terms of run-pass tendencies and aggressiveness. So, you can't really know which situations I'm going to be aggressive in and which I'm going to be conservative in. I may, in fact, include both in the same down and distance. If you commit too heavily on defense in one direction or another, my guys will eventually stumble upon the mismatch and I'll profit from it. In that same regard on defense, unless I can overwhelm you based on a talent mismatch, I prefer to force you to earn everything you get rather than over-committing and getting caught with my pants down.