My mental picture of Delta-X Criterion is that when the curve is flatter (less development), you would print on a contrasty paper. The salvage grades of paper would amplify the shadow detail that would otherwise be inadequate on a normal paper. So you can get more out of the thin shadows.
Originally Posted by AndreasT
The converse happens when you develop the heck out of film. Once you decide to print on a flat paper, that paper needs more shadow separation, so you need to provide more shadow exposure.
The end result is that the speed doesn't change that much with development changes.