لَيْسَ كُلُّ مِيزَانٍ مَعِيبًا.
Not every scale is defective; some scales are sound.
لَيْسَ فِي السُّوقِ مِيزَانٌ مَعِيبٌ.
There is no defective scale in the market.
The first is negation of universality: it denies that all scales are defective, consistently with the two observed faults. The second denies any defective scale at all and therefore conflicts with the reported findings. The scope of negation determines the substance of the claim.
When a listener generalizes an inspection to all merchants, the first formulation corrects that overgeneralization without pretending that no fault exists.