Quite old this topic but useful information now I am working on this.
I found out that a Solid created by the AutoCAD Command creates a more intelligent 3D Solid than I create with code. Do a list on both and you will see this:
3DSOLID Layer: "0"
Space: Model space
Color: BYLAYER Linetype: "Continuous"
LineWeight: Default
Handle = 145
History = None
Show History = No
Bounding Box: Lower Bound X = 159.809 , Y = 159.747 , Z = 0.341
Upper Bound X = 198.179 , Y = 186.957 , Z = 4.638
3DSOLID Layer: "0"
Space: Model space
Handle = 157
History = None
Show History = No
Solid type = Sweep
Sweep length: 38.867
Profile rotation: 0.00
Scale along path: 1.000
Twist along path: 0.00
Press ENTER to continue:
Bank: Off
Bounding Box: Lower Bound X = 173.891 , Y = 144.954 , Z = -0.692
Upper Bound X = 205.943 , Y = 169.206 , Z = 0.692
So, the API probably just create a 3D Solid and AutoCAD can create a more intelligent object.
Also, and that makes it really weird, the coded solid get XData added! Why?
* Registered Application Name: ACAD_STEPID
* Code 1071, 32-bit signed long integer: 2