If a wheel is not perfectly round, the linear speed of the robot varies even as the wheel turns at a perfectly constant rate. If a robot uses differential steering, uneven speed translates to turning (slightly) and therefore changing direction (slowly). The robot will travel in an `S' shaped line.
Even if a wheel is perfectly round, you can still experience uneven linear speed as the wheel turns at a constant angular speed. This is because the axle may not be at the center of the wheel. This also causes the robot to travel in an `S' shaped line instead of a straight line.
If the two wheels of a differentially steered robot are different in size, the robot will always skew to one side (the side with the smaller wheel).