I don't know the answer to your question but I would guess that when all bodies are selected, the deformation is plotted at whatever location has the maximum deformation at each frequency.  I don't find such a plot very useful.

I always select a vertex to plot the directional or total deformation versus frequency in a Harmonic Response analysis so I know exactly where the amplitude is being reported.

From that FRF plot, I note the frequency of the peak response and request a Stress plot at that frequency so I can see where on the structure the peak stress can be found.