Anandtech - Inside the XBox 360 article

By the way, Bill, the actual number of transistors in Xenos dedicated to logic is actually about 252 million (see the B3D article). It's about 100 million for the daughter die with edram included, 20 million of these dedicated to ROP logic. So it all comes-out to about 232 million transistors dedicated to logic on the main die and 20 million on the daughter die. Remember, as stated before, there is some additional hardwrare in Ati's pc gpus that is present only for legacy purposes. In addition to this, in the pc space, is sometimes extra functionality for 2D/video processing, which shouldn't be required for such processing in Xenos, since it can, most likely, be emulated by its rendering resources.

 

I think the reasoning behind this is that there are other things besides the 16 pipelines, which are configured differently than Xenos...

It also has media acceleration protocols in chip among other things...