Code:
# ———– place the box i on the convegor ——
# approach to the conveyor
robot.setPoseFrame(null_position_holder)
robot.MoveJ(target_null_safe)
robot.MoveL(eye()*roty(-pi))
You were applying the distance between the robot base and the NullPositionHolder frame twice.
Once with the setPoseFrame and once with the motion. The pose you were trying to reach was therefore twice as far.
85 -> You set the frame, so the new (0,0,0) is where the frame is.
86 -> You take the frame position with respect to its parent, which represents the same translation as what you did on line 85.