Probably some minor differences in the way the shoulders and collars are rigged. Try tweaking the Walk Designer's Arm Out slider and see if that helps.

If not, you might have to create a custom walk cycle for the robot. After applying the Power Walk pose from the Library, adjust the arms, frame by frame, until the animation looks right, then resave the animated pose to the Walk Designer folder under a new name. The next time you use Walk Designer (you might have to relaunch Poser first), the new walk cycle will appear as a Blend Style option in the control panel. Set that to 100% and it should work.