One, the main reason, power and steering through 2 wheels = them to spin, when they spin they lose traction, thus causing the car to go in a straight line rather than turn.

Negative camber will only work in the dry when you're taking a mediumish corner pretty hard and well as it makes the whole tyre sit on the road, in the wet, round a roundabout, i'd be suprised if it wasn't aiding the understeer.

Fact that its a mini with tiny wheels also won't help.

Although in the wet, round roundabouts, I seem to get far more oversteer.