Its all due to the suspension setup or "Roll induced camber"

Think of the car rolling / tilting as it goes round a corner, best grip comes from the tyre tread being flat on the road, if the wheel is angled slightly when the car is going straight then as it goes round a corner and the body rolls, the tread becomes horizontal with the road. If the wheel was vertical then the tread would go past horizontal and result in less grip.

Usually you can't see the camber, it shows up more on car that have been lowered - the old BMW 3 series is a classic, a boy racer buys one, lowers it far too much, the wheels are at a horrendous angle, they get less grip, but hey, the back end steps out more.

Hope this makes sense! I could explain it a lot easier with pencil and notepad. (12/99)