Its all to do with physics, and whether those are properly calculated into the game. Its a very very tricky process and it has to be done by someone who is very intelligent. i racing good example of a simulator, the PC flight sims now qualify towards FAA flight hours.
In real life the bumps cause the cars to loose grip, along with the tires being different. The drivers are manipulated by G-forces, which are actually tough to deal with during braking. It can cause an out of shape "joe six pack" to pass out in the car. Watch the 2 seater F1 car at Abu Dhabi, the reporter guy almost barfs. F1 drivers are paid alot, because not many can push the car without making mistakes, and handling the G-forces at the same time. If you make a mistake (like reventon did a couple nights ago) the car is trashed and its millions of dollars or Euros.
GT5 physics are not up to snuff with iracing physics. The dimensions and little details of the Ferrari F1s that PD plugged in are not going to be spot on. Also their "engine" has no been released to the public and so nobody knows how good it acutally is...
Other games like F1 2010 have no physics engine, and no realism. They manually determine how the cars drive. Have you ever noticed in that game you feel like your on thin air.....like your not touching the road. Its because there's no physics, and no downforce.