This is because the formula for the length of an arc uses the angle in radians.
For $180^{\circ}$, we have $\theta = 180^{\circ} \times \frac{\pi}{180} = \pi$ radians.
For $60^{\circ}$, we have $\theta = 60^{\circ} \times \frac{\pi}{180} = \frac{\pi}{3}$
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