But it looks like we all could have been fooled, myself included. It appears Microsoft is using a multi-module, stacked design in the Xbox One. Underneath that modified Radeon HD 6670 GPU, lies a 175 mm^2 space for something. That something is presumed to be a Volcanic Islands based AMD with a 22nm SOI design featuring 2304 to 2560 stream processors.
But it's the second reason which is brilliantly clever. The Volcanic Islands GPU wasn’t officially announced until the last week of September in Hawaii; which means Microsoft couldn’t list it as a specification. As for why they don't reveal now? It's possible a NDA is in place that could prevent talking about the GPU until after retail release.