Red Castle
Adventurer
If by NADs you mean Fortitude, Reflex and Will, while going by the formula to create monsters it was effectively usually 2 points less than AC (depending on monster role), in practice, just by looking at monsters from the Monster Manual and Vault, it was more complex than that. NAD were used to give more flavor to creatures. Your creature is skinny but fast? it will have a lower fortitude and higher reflex; mentally strong like a Mind Flayer? expect it to have a strong Will.First of all there's a 2 point gap between NADs and AC, which is utterly pointless. It was obviously put in with the theory that it corrected for weapon proficiency, but all they needed to do was add implement proficiency that worked the same way instead.
Sometimes, it could even go higher than the AC, like an Owlbear with AC20 and Fortitude 22... sometimes much lower, like the Minotaur with AC 24 but a Will of 19. It was interesting because it gave a layer of tactic, trying to find if the creature had a particular weakness. Sometimes you could just guess it by using logic (Kobold probably has a lot of reflex but not a lot of will since they are mostly cowards), sometimes you could make a Nature or Dungeoneering check to see if your character knows that kind of creature.