I dunno Dan, I think they tested both modes:
Take a close look at that chart. One line has "Native HD Graphics, Virtualize Discrete Graphics" (I MODE) while the other has "Native Discrete Graphics, Virtualize HD Graphic" (D MODE). Both have (roughly) the same power consumption at idle.
It's possible that the Virtu software isn't capable of deactivating all GPUs. It could simply be that what we are seeing is a lack of maturity in the software and the technology in general. I don't have an answer for that right now.