The CAESAR II XJ Modeler produces a flexible element with length. While the hinge plates restrict the motion of the joint ends (to rotate about the pin), they do not necessarily eliminate lateral and axial movement of the flexible element. I would have to think that the lateral and axial motion is quite small in your model.

For many years, most XJs were modeled as points in a piping system. If modeled as a "zero length" XJ your results would not show this relative lateral and axial movement.