Jeff, me reply below is based somewhat on speculation of Jon's piezography profiler, but also on my experience making my own external linearization process from the piezography master curves (i wont go more into this now). From what I've experimented with, the piezrogaphy master curves are designed to lay down too much ink (and they block up in the shadows). When the 256 step target is printed they can see at which point the paper blocks up and that point becomes the high limit. It is like setting the output shadow slider in a photoshop levels adjustment. and all the the x values are adjusted accordingly ( If you open some of the piezography curves in a text editor some will even say "limit 95.0"). When they run their linearization function the y values are adjusted up or down and to arrive at the needed y value or each ink to create the needed tone. So when I said the x- values don't change it was not entirely accurate, they do change based on whatever the overall limit is at the end of the scale, but the change is not dramatic.
Also take a look at the differences between the P2 K6 matte and K7 master curves: they are the same except the k6 curve doesn't have the shade 7 ink. Since the x value for shade 6 in both the k7 and k6 curves starts at 0,0 and immediately begins increasing, the y values are only shifted up to create the darker tones needed in the highlights of the k6 curves where those tones were covered by shade 7. The benefit of the really long overlaps along the x axis is that y vaules can be moved up or down for each ink at more points than with the QTR partitioning method.
Richard Boutwell