The defining property of a bell curve is that it shows the probability of finding a given value, as a function of the value, and that this probability distribution is shaped like a bell (hence the name), with the highest probability found at the average value.

I get that channelers are weaker in the Third Age, and are mostly found in the bottom half when using the same scale as during the Age of Legends. So while you can certainly use the same scale to discuss the strength of channelers in the Third Age and in the Age of Legends, it makes no sense to talk about the same bell curve, because the distribution of strengths are not the same.