What was interesting here was that the turbo feature has two frequencies - one with AVX instructions and the other without. When running AVX instructions the frequency drops to 1.9GHz. Without AVX instructions it is 2.3GHz. This goes to say how carefully Intel has had to pack these cores into that power budget. In certain applications the previous IvyBridge 10core and 12core parts have higher performances as they can clock higher.
@colin is right. Intel went with a ring based interconnect from sandybridge and that makes it easier to come up with odd number of cores. I am sure 18 was as far as they could push the core count without blowng up the socket power budget. The part is rated at 145W but apparently burns close to 165W in reality.
It seems that 18 was chosen because the layout allowed it. Here is what Intel's chief architect says: "The internal micro-architecture is not constrained to a power-of two. Implementing the layout as four columns (three with four cores and the fourth with six cores) allowed us to efficiently use the available silicon area and provide the maximum number of cores. "