This kind of flies in the face of that “believe in your own product” mantra, but to be fair this was the first showcase that took place some 6 months before the console was ready and there are no indications that things won't work as advertised at launch.
The HP system with Windows 7 was not as surprising as the choice of GPU, the Geforce GTX that you can see on the second picture in the link here, with a GTX logo in tow. Someone even mentioned that the HP based PC had Intel an Intel inside, but we cannot confirm or deny it. Of course, the Xbox One has AMD APU that includes CPU and graphics on the same silicon.
This particular development team at E3 used an Nvidia card, GTX 770 or 780 as the picture has been shot from a distance, but even if we talk about GTX 770 this card is much faster than the Radeon HD 7790 equivalent that is a part of AMD’s custom Xbox One APU. This might not be the best representation of that is to come as the graphics in the demo kit is tremendously faster than the one that will ship with Xbox One.
Many developers still use Nvidia tools to develop high end games and this might also be the reason why the PC had an Nvidia card inside. We heard from some developers that they develop the game on Geforce and then at a later date they make sure that it all runs smoothly on Radeon and in this particular demo that seems to be the case.
It looks like some developers still develop on Geforce powered platforms hoping to migrate to Radeon based development kits at a later phase of development. This is not a singled out case, our correspondent at E3 heard that from a few developers.