That's an excellent question.
Essentially, we have two systems of law in these two different fora. Under the WTO, the law is an international treaty. For the WTO agreements in particular—the subsidies and countervailing measures agreement, and the WTO anti-dumping agreement—treaty text is interpreted under international law.
Under NAFTA, chapter 19, or CUSMA, chapter 10, what you're looking at is the application of U.S. law. Often, the two systems are very similar. For example, on the subsidy side, the main provision under the WTO is article 14(d) of the SCM agreement. It's United States code 1677 under U.S. law. They're the same, almost word for word.
Obviously, there is different case law that's come out of the two systems. Under U.S. law, there is jurisprudence that's developed by the court system, and the WTO has its own jurisprudence that has been developed. In some cases, it might be better to go before the WTO on an issue. For example, on dumping, there's something called “zeroing”, which the WTO has found to be inconsistent, whereas the U.S. courts have traditionally found to be consistent with U.S. law.