I have no explanation for the difference other than possible patent issues. I'm just pointing out, ArtCAM http://www.artcam.com/ uses ALL 3 methods listed above( color assignment, grayscale and "puffing" ). ArtCAM reliefs are saved as (.rlf) files. Another 3D software package you might want to look at is MasterCAM Art http://www.mastercam.com/Products/Art/default.aspx At the time I was choosing software MasterCAM lacked a lot of features. But the latest version looks to be coming along.
It all just depends what you plan on doing and how much $$$$$ you got to spend. They all have their pro's and con's. I really like ArtCAM. But, as you probly figured out, there are a lot software packages that will do pretty much the same thing. Even then "pretty much the same thing" is not the same thing. Any software is only as productive and the end product only as unique as the users imagination and skill.
Cut3D would be another relief based 3D CAD/CAM. http://www.vectric.com/