OK, let me rephrase the question.
Where should I place the machine datum for this rotary operation to make the simulation work correctly ?
By default the machine datum is placed at the intersection of two rotary axes (in my case A and B). But I think Pathrace moved the machine datum to some other position in their example in order to make the simulation work... this is what I believe.
How do you work with this kind of rotary operations ? How is your simulation working ?