Because the specification can be just complex as the system
That's true in principle, but in practice I don't think it's going to hold in most cases. A good specification necessarily captures the essential complexity of the system, but it's a lucky developer whose implementation doesn't introduce considerable accidental complexity as well. Ensuring that this accidental complexity doesn't break anything could have great value, depending on the nature of the system and how damaging a failure could be.
That's true in principle, but in practice I don't think it's going to hold in most cases. A good specification necessarily captures the essential complexity of the system, but it's a lucky developer whose implementation doesn't introduce considerable accidental complexity as well. Ensuring that this accidental complexity doesn't break anything could have great value, depending on the nature of the system and how damaging a failure could be.