It's in the ECU tuning and intake/exhaust restrictions. It's also company politics. The 350Z is the flagship Nissan sportscar, so has 287. The G35c came next and is 280 (it's in the quieter intake design) b/c they didn't want it to match the Z (even though it's the Skyline in Japan; the Coupe was released here in the US first). The G35 sedan is at 260 b/c of more restrictive exhaust, ECU tuning and the fact that they needed the coupe to boast more power to be sportier. The FX has the same set-up as the coupe b/c it's a sports car with a backpack!.