Why does SWI-Prolog invent f/2 given only f/1?

Question

SWI-Prolog 7.6.4:

?- dif(X, f(Y)), X=f(a).
X = f(a),
dif(f(f(a), Y), f(f(Y), a)).

Note that I use f/1 in the query, but the cons