Wind resistance and weight of a vehicle have absolutely no bearing whatsoever on how much HP gets delivered at the drive wheels of a particular vehicle. They do affect the level of performance you can get for a fixed amount of power. An example of this is the LS1 that's available in both the Corvette and Cadillac's new sedan. While not identical, the motor's are very similar in output. But the performance of the two vehicles is quite different due to (among other things) the different weight and aerodynamics of the two bodies they're in. On the dyno - their outputs should be similar.
The factory rating approximates the power output -- but in reality there is a wide range of power outputs from factory produced engines because no 2 are identical. The testers in magazines will tell you that it's not uncommon to have as much as .5-.8 seconds difference between two identical models of the same car. Some come out stronger than others - it's a fact. So all the factory number does is give you an idea of the HP/torque levels and the rpm at which they occur -- that the designers were shooting for.
The factory rating approximates the power output -- but in reality there is a wide range of power outputs from factory produced engines because no 2 are identical. The testers in magazines will tell you that it's not uncommon to have as much as .5-.8 seconds difference between two identical models of the same car. Some come out stronger than others - it's a fact. So all the factory number does is give you an idea of the HP/torque levels and the rpm at which they occur -- that the designers were shooting for.