There are several writing tropes oriented around misinterpreting prophecies or signs of "Chosen Ones" that may give you some ideas.

• Prophecy Twist - The prophecy comes true as it was written, but in an unexpected way, and the signs didn't mean what everyone thought they meant. The classic example is Lord of the Rings, when the Witch-King confidently declares that no man can kill him... only to be killed by a woman.

"Hinder me? Thou fool. No living man may hinder me!"

Then Merry heard of all sounds in that hour the strangest. It seemed that Dernhelm laughed, and the clear voice was like the ring of steel. "But no living man am I!”

Perhaps in your story, the character who thinks he's the "Chosen One" believes that because the prophecy lines up with him so well, it means it must be him, when in actuality the clues meant something completely different and were easily misinterpreted. Maybe the prophecy said the hero would be born on the fourth of Oktumber, and your false Chosen One is confident that's him, because it's his birthday... when in actuality, that's the day the true Chosen One's village was burned down and he decided to become a hero, thus making it the day of the true hero's "birth."

• Prophetic Fallacy - The prophecy is incomplete, or outright deceptive, or told by a liar. Or, alternatively, the people who heard it, including the hero, heard it wrong.

To quote TV Tropes:

For example, a man might see himself being knocked down by a car and note that the time on a digital display is 10:51, then spend the entire episode trying to avoid going near a road, despite various events conspiring to put him in danger. He eventually makes it to 10:52 and thinks he is safe, but is knocked down an hour or so later and discovers that he saw the digital clock in a mirror and his actual time of death is 12:01.

Maybe in your story, the prophecy was incomplete and parts were missing. The false Hero believes that he's the one, and so does everyone else, because he doesn't have the missing parts that reveal the full prophecy, which actually lines up with the true Chosen One.

• Prophecies Are Always Right - The prophecy is always right... or at least that's what everyone believes.

Perhaps in your story, the prophecy is seemingly ironclad, and so your false Chosen One is confident that it's him because he matches it perfectly. When in reality, the prophecy was just... wrong. It wasn't the true prophecy, or that's not how fate works, or it was just a big scam by some divine entity to mess with people. Whatever the case, the true Chosen One doesn't seem to fit, and so that's why nobody suspects they are the real Chosen One.

There are several writing tropes oriented around misinterpreting prophecies or signs of "Chosen Ones" that may give you some ideas.

Prophecy Twist

The prophecy comes true as it was written, but in an unexpected way, and the signs didn't mean what everyone thought they meant. The classic example is Lord of the Rings, when the Witch-King confidently declares that no man can kill him... only to be killed by a woman.

"Hinder me? Thou fool. No living man may hinder me!"

Then Merry heard of all sounds in that hour the strangest. It seemed that Dernhelm laughed, and the clear voice was like the ring of steel. "But no living man am I!”

Perhaps in your story, the character who thinks he's the "Chosen One" believes that because the prophecy lines up with him so well, it means it must be him, when in actuality the clues meant something completely different and were easily misinterpreted. Maybe the prophecy said the hero would be born on the fourth of Oktumber, and your false Chosen One is confident that's him, because it's his birthday... when in actuality, that's the day the true Chosen One's village was burned down and he decided to become a hero, thus making it the day of the true hero's "birth."

Prophetic Fallacy

The prophecy is incomplete, or outright deceptive, or told by a liar. Or, alternatively, the people who heard it, including the hero, heard it wrong.

To quote TV Tropes:

For example, a man might see himself being knocked down by a car and note that the time on a digital display is 10:51, then spend the entire episode trying to avoid going near a road, despite various events conspiring to put him in danger. He eventually makes it to 10:52 and thinks he is safe, but is knocked down an hour or so later and discovers that he saw the digital clock in a mirror and his actual time of death is 12:01.

Maybe in your story, the prophecy was incomplete and parts were missing. The false Hero believes that he's the one, and so does everyone else, because he doesn't have the missing parts that reveal the full prophecy, which actually lines up with the true Chosen One.

Prophecies Are Always Right

The prophecy is always right... or at least that's what everyone believes.

Perhaps in your story, the prophecy is seemingly ironclad, and so your false Chosen One is confident that it's him because he matches it perfectly. When in reality, the prophecy was just... wrong. It wasn't the true prophecy, or that's not how fate works, or it was just a big scam by some divine entity to mess with people. Whatever the case, the true Chosen One doesn't seem to fit, and so that's why nobody suspects they are the real Chosen One.