_"In other cases, such as
?- X \= Y., the predicate fails although there are solutions."_
This is a bit unclear.
The question being asked is _"is there no evidence that terms X and Y can be unified (at this point in time)"_?
(alternatively _"if there is any way that terms X and Y can be unified, fail!"_)
So "false" is arguably correct as an answer.
The reading in classical logic (or even constructive logic?) would be
_"in any solution, X and Y must be different!"_
and the correct answer would be "I don't know either way at this point in time, let's revisit this when we know more" ... hence "dif".