See:

http://www.darkroomautomation.com/su...vcworkings.pdf

In general, changes in contrast in the lower grades are in the shadows while the contrast changes in the higher grades are in the highlights.

The 'bump' in the middle of the lower VC grades is an intrinsic property of VC paper. The difference you are seeing is because the two heads are printing at slightly different contrasts because they have different spectral curves.