Maybe the dynamic nature of the LOD system explain the moderate polycount of the first photo. There are more objects on screen apart from the car; other cars, pilots, pit stop crew and so on... I suppose that the (subjective) high quality of the models in-game in Prologue is a bit tricky: the cars are substantially far from yours at the start of the race, and the polygon budget is expensed mainly in your car. The image shown before is substantially more demanding.
In fact, I think Polyphony masters the art of pushing the tech just in the best direction, in order to make its games as much photorealistics as possible. And it´s obvious that the hardware is the same that runs Dirt or F1 CE, it´s not arcane magic; just optimisation.
In any case I´d like to know the real amount of polys of the cars that Nebula posted. Of course I respect the abilities of professo estimating the polycount, but I´d like to know the real data.