All known nobles I have tested (and by "known", I mean completely reliable accountability from plant-in-ground to product-in-bag) give readings within this narrow range, in spite of variations in cultivar, age, soil, root/rootstock ratio, and cultivation technique.
In addition, their reaction to adulteration is nearly identical regardless of initial lightness point, and a saturation point is reached that follows a much different curve than other potentially adulterated samples. (This also indicates that trials on samples above the range are effectively adulterating already spiked kava, but that is a subject that would take much more explanation.) This indicates to me - and to experts I have offered this theory - that the range is reasonably accurate, and that the
molecule responsible for the coloration is unique.
The chart below only shows a few of the samples tested, but illustrates the theory. Note the narrow range of the verified nobles: