Disjunction G1 ; G2 vs. If-then-else Cond -> G1 ; G2

I encountered a Prolog program containing a nested if-then-else of the form

p(X,Y) :-
     (cond1(X,Y) -> q(X)); true,
     (cond2(X,Y) -> q(Y)); true.

that had unexpected answers. The reason for this behaviour is the same as in the following queries:

?- (true, (true -> X=a)); X=b.
X = a ;
X = b.

?- ((true -> X=a), true); X=b.
X = a ;
X = b.

?- (true -> X=a); X=b.
X = a.

The first query has two answers but the second one has only one.

P.S.: I know the difference, but I have not found a SO question that deals with this quite confusing phenomenon. So why not have it documented this way?