If we look at the strings of applications to b, we have f →d ( f ←a ( f →c (b)) for the reference tree and f →d ( f →c ( f ←a (b)) for the candidate; in counting the wrong choices made, we see that we made one wrong choice of applying f ←a instead of f →c ; counting such wrong choices would assign the same distance to the candidate tree as would δ SH in this case.
δ SH and δ ouas agree.