When the robot is turning, wide tires result in errors because the traction surface is large. For turning, the ideal contact surface should be a single point. This is because a wide tire contacts the surface in a line, and different points of this line (as concentric circles around the turning pivot) want to travel at different speeds. However, because the wheel is driven by a single axle, all points of the tire surface must travel at the same speed. This means some parts of the contact surface have to skid, and skidding always introduce errors.
Note that with traction related errors, a robot may travel more than what it intends, or it may travel less than what it intends. This is because you get errors during acceleration (which shortens the actual distance), and you also get errors during deceleration (which lengthens the actual distance).