We read:

_"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".