Whenever people wonder why the leaders are 10 seconds faster than themselves, the answer is usually found between their ears. Most of the hackers in RR3 are not clever enough to make a modest time, so they make impossible times which are obvious hacks. But if the time is within reason and it looks like many other people are coming close to it... take a look at this video and you may figure out the problem. 

http://www.youtube.com/watch?v=m-DNkvGni4k

(can't find a way to embed videos on this site, so had to put a youtube link)