You craft a set of new armor. From that moment on, it starts to decay rapidly, even if your not in a battle. After one dungeon run, it has decayed significantly, or if you were engaged with any hungry critters, it might be reduced to useless levels of protection.
You craft some food. From that moment on, it never decays, even if stored in a pack for months. If you use it in a dungeon run, say a tart that boosts your Magika for one hour - it continues to provide that level of Magika boost for the full hour, regardless of how many hungry critters you fight.
Does this make sense?