Just some thoughts.

If you are using a video card for display and a GPU render, the reported memory difference could be correct due to hardware overlays that are already in video memory prior to launching the render engine. You would be shocked at how many programs grab video memory as soon as you open them. They have too, they are coded to use hardware overlays for the display.

Not using the rendering GPUs for display purposes will also allow programs written to load the cores effectively to work as they should.

To be fully effective, each GPU core has to be fed as many things as it can handle at once. It is not easy to get a GPU to be fully effective and at 100% load, adding the GUI to it drops that chance to all but zero. Most GPU render engines are coded to be the soul thing running on the GPU, take advantage of all the GPUs cores, and feed each core what it can do at the same time.

Based on the speed that Octane renders, it must be optimized extremely well, and taking full advantage of ordering and loading of the GPU cores.

If you can use a different GPU for the GUI, by all means do so. It will render faster, wont interfere with the GUI, etc..

You can even set up different hardware profiles to boot too if you need to use your good GPUs for games, editors, etc.

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