What is the reason for not being certain about the realization of a vision, and is that due to its interpretation being conjectural rather than certain, or are there other reasons despite the existence of a certain interpretation?

Dreams are interpreted according to their meaning, provided the interpretation comes from someone knowledgeable in dream interpretation. Not every dream comes true or can be interpreted, as many are mere jumbled dreams. Scholars have differed on whether the interpretation of prophets' dreams is speculative or definitive. Al-Qurtubi leaned towards it being definitive, because what the prophets say is based on revelation. The Prophet's (peace be upon him) saying "If God wills" does not negate the definitiveness of the interpretation, as it may have been said for the sake of seeking blessings.