This tutorial (page 22) on Hopcroft-Karp algorithm for maximum bipartite matching states the following:
Let $M^*$ be a maximum matching, and let $M$ be any matching in $G = (V, E)$. ...
Let us consider the graph $G'=(V, M \oplus M^*)$. It contains at most $|M^*|-|M|$ augmenting paths with respect to $M$.
Here $M\oplus M^*$ is the symmetric difference between $M$ and $M^*$. But how does the last line (It contains at most $|M^*|-|M|$ augmenting paths with respect to $M$) follow?