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 differencesamong purchasers, regions or time periods.
It even says that there are huge differences within some regions, pointing out that in north west England, a 38% poverty level among children living in Manchester can be contrasted with a figure of 7% in Ribble Valley.