Except that in those cases the product is clearly marked as a 4 pound bag of sugar, not as a 5 lb bag, which would not be the case if vendors are selling wire smaller than 12 gauge as 12 gauge.
You're not thinking clearly. Just because the insulation layer is thinner (better thermoplastics) on newer wire, doesn't change the amount of copper in the wire.
The claim is bogus.
It is true, however, that the thermoplastic coating on modern THHN & NM is not as bulky as it used to be, due to using newer formulations with better insulating qualities.
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