Road supercars vs. race supercars

[sssssssss]

I'm quite confused as far as the main principle that makes a racecar so much faster than a road car is concerned. Which are the main factors that make the racecar lap a track so much quicker than a roadcar? For instance, you'd expect a car like the Bugatti Veyron, with its 1001 hp and 407 km/h top speed, to be comparable to an LMP1 car, or at least to a 1995-1998 era GT1. But if you compare its times on let's say Nordschleife to that of a Porsche 956 of the 1980's, it's 1 min 30 sec slower! Why is that? Because if you compare the performance statistics, the Veyron seems to beat many racecars: 1001 hp, even with 1800 kilos, is comparable to 600 hp at 900 kilos. And it's not just about the Veyron, it's about the whole elite bunch of road sportscars. This was just an example.
For a while I thought the acceleration might be the main factor, but the LMP1's don't reach 100 km/h in less than 2.5 seconds... I guess the Ferrari 333SP was declared to hit 100 km/h in about 3.1 seconds, from 0 km/h.