Coaxial cable test with VNA to estimate Wave Velocity and Attenuation
With the far_end shorted, the VNA will show a notch (or peak) at some frequency proportional to the length.
The depth of the notch, or height of the peak, indicates attenuation.
From what I recall of such measurements (discussed in the literature or the equipment manuals), an OPEN is easier to achieve than a SHORT. The SHORT needs 0 inductance, which is impossible given the finite length of wire needed to short the center pin to the shield.
Thus your instructor probably wants you to learn about that (small distance, but not zero) SHORT as an error source.
Have fun.