Wild speculation here, but what if the numbers and rumours are correct and it's 9 vs 12 but the 9 doesn't include AMD ray tracing?
Maybe Sony has their own RT solution that they'll add to the gpu bringing the overall level closer to 12 while microsoft are using AMD RT and so the gpu appears to be more powerful at first glance.
Second piece of even wilder speculation, as DXRT treats RT as shaders is it possible that the series x gpu doesn't have dedicated cores as such, but instead all of the shaders are fully capable of needs be? Kind of like an evolution of the unified shaders introduced in the 360.