You can't see, that L decreases with frequency. It's just a method to calculate inductance from admittance measurements. The behaviour is simply from Y = 1/jωL.
Let's take a finite-length coaxial cable as an example. The coax cable stretches from point A to point B. The coax consists of inner core and outer core which are both made of metal.
Now, let's agree that the inductance is associated with magnetic flux. Imagine there is a magnetic flux entering the inner core at point A and goes out at point B and enters the outer core at point B and goes out at point A and enters again the inner core at point A. Thus, we have a loop here.
At high frequency, skin-effect will occur, meaning the current will tend to approach the surface of the metal (of course exponentially), which is in this case getting closer to each other. The current inside the inner core will go outward and the current inside the outer core will go inward, causing the magnetic flux loop become smaller, hence, smaller inductance.