Some of those comparisons produce positive dumping margins (whenever the U.S. price is lower than the home market price) and some yield negative dumping margins (whenever the U.S. price is higher than the home market price).
The justification for the exception under Article 2.4.2 is, presumably, that that comparison methodology allows the authorities to home in on targeted dumping, which may be revealed by patterns of price differences among purchasers, regions or time periods.