For example: - suppose a body covers a semi-circle in $5$ seconds, then distance$=\pi * r$, where $r$ is the radius of the semi-circle. Displacement is $2r$ only. Then the value of speed is $[\pi * r] / 5$ while that of velocity is $2r/5$. Here we can see that magnitude of velocity is different from that of speed. Is it correct?
Instantaneous speed always equals the magnitude of instantaneous velocity (because the instantaneous displacement is small enough to be regarded as straight-line). In your example, you are comparing average speed and the magnitude of average velocity - and the two of them can be different as you have correctly calculated.