Traction control ALLOWS some slip between the two wheels, this is to do with the way that the tread blocks move on the driven wheel versus the undriven wheel.

Consequently there is an optimum amount of slip for any given condition, but not one figure thats optimum for all conditions.

For example if you are drag racing in the wet, you allow less slip than in the dry, and the tread blocks move less due to having less grip.

If you are on slicks you allow even less again.