its the different tracks. you have to take into account the banking on the turns, the smoothness (or bumpiness) of the road, varying entry, drift, and exit speeds, throttle and countersteer modulation, the list could go on and on.....
Also dont expect to be able to just go to a track and drift, you have to practice and learn every inch of the track. Take your car out and do laps until you run out of gas, that should get you started on learning the track.
I can currently only drive 1 track without mistakes, Trial Mountain Reverse.
If you are troubleshooting oversteering/understeering/etc.. problems with your car, check out some of the links in my signature and print them out. They have helped me out alot in fine tuning some of my cars. 👍