My question is not about the definition of the two rank correlation methods, but it is a more practical question: I have two variables, X and Y, and I calculate the rank correlation coefficient with the two approaches. With the Kendall-tau-b (which accounts for ties) I get tau = 0 and p-value = 1; with Spearman I get rho = -0.13 and p-value = 0.44.
Why does Kendall tell me there is exactly no correlation, while Spearman does see a nonzero correlation?