Peter,

That curve is simply representative of a certain set of conditions;

image.jpg

Use something besides Ilfotec HC developer, change the dilution, modify the agitation scheme, adjust temperature, or adjust any combination of the factors and you get a different curve, each one unique.

Ilford's ID-11 curve actually shows to shoulder sooner.

image.jpg

The published curves I've seen all seem to fit in a given box, they show the toe and what I might call the normal exposure print range. Kodak is a bit better than Ilford about showing alternative condition curves but they still don't show what happens farther to the right. We simply have no info from Ilford about what happens with more exposure.