Maybe you miss out the following description:

choosing a single number and moving it some number of positions to the left.

You can't move number to the right.

Your example is too long, I tried your solution and found a way shorter example.
Given a = [3, 2, 1] and b = [2, 1, 3]. Your solution returns 1, but obviously the correct answer is 2.

This kata is Python only at the moment I'm writing.
Given some users already found the correct solution, there is an issue in your solution, not in referee solution.

Why? Design an example which this code will fail.

Approved, thanks