And the rationale would be? C'mon... Talk nerdy to me. 
And Bridge, if you're asking about the rear wheel being 19" in diameter vs 18" at the front, I'm not really sure. Mathematically, upping the wheel size an inch over the 18"er means about 4.4% more speed over the road at the same RPM. So if top speed (without electronic intervention) was 250 kph on the 18s, you could squeeze it up to 261 on the larger wheels. Although the extra aerodynamic drag would grab some of that back. At more reasonable speeds, that would translate to lower engine speeds for a given vehicle speed, which would have fuel economy implications...certainly an issue in places like California.
So why not up the size of front ones too? The Carrera S is running 19s all 'round. My guess is it's a styling thing....keeping the nose low and maintaining that dramatic wedge profile.
Or maybe I'm just talking out my arse.