Prolog, result correctly given but false is printed

Question

I have for example the following function computing the set of all internal nodes in a binary tree (nodes which have at least one child, i.e. are no leaves)

internals(nil,[]).
internals(t(_,nil,nil),[]).
internals(t(X,L,nil),[X|S]) :- L = t(_,_,_), internals(L,S).
internals(t(X,nil,R),[X|S]) :- R = t(_,_,_), internals(R,S).
internals(t(X,L,R),[X|S]) :- L = t(_,_,_), R = t(_,_,_), 
   internals(L,SL), internals(R,SR), append(SL,SR,S).

the result of this is

?- internals(t(3, t(2, t(1, nil, nil), nil), t(5, nil, t(7, nil, nil))), X).
X = [3, 2, 5] ;
false.

Why does it end in false and how come the following code does the same but does not result in false

internals(nil,[]).
internals(t(_,nil,nil),[]) :- !.
internals(t(X,L,R),[X|Xs]):-
    internals(L,LI), internals(R,RI),
    append(LI,RI,Xs).

?- internals(t(3, t(2, t(1, nil, nil), nil), t(5, nil, t(7, nil, nil))), X).
X = [3, 2, 5].