Let me just remind everyone that grade levels ahead based on being able to do work from actual courses, and grade levels ahead based on grade equivalents from a standardized achievement test are two very different propositions, only one of which has significant validity--and it's not the one from the NRT.
And yes, the developmental arc for reading is very different from that for mathematics, and less dependent on access to direct instruction. (One of the reasons not to lean too heavily on grade equivalent scores is the early plateau in reading decoding raw scores.) The change in slope across the lifespan for academic growth also means that 2-3 years ahead at one point in time has a very different meaning from 2-3 years at another point.
Plus, in mathematics, pretty much the whole continent of North America is about 1-2 years behind the rest of the industrialized world, which I don't think is due to the population being a standard deviation lower than the rest of the world in intelligence (though there are moments when that seems rather plausible!).
