If you haven't read the comic The Search, I advise stopping reading, because I will reveal a big spoiler (hidden).
During many episodes of the third season of ATLA (Book 3: Fire), Fire Prince Zuko is not well. He can't understand the reason, and he can't ask his uncle because he's mad at him, and Iroh is in prison.
When he receives vague mysterious clues, and he has no other choice, he goes to see his uncle in prison, finally uncovering the truth of his illness: the reason he feels that way is that he is a descendant of two very conflicted and powerful bloodlines - Fire Lord Sozin from his father's side, and Avatar Roku from his mother's side, and this conflict is now raging in him.
Of course when he realizes that, he magically gets better when he chooses the "right path".
So far so good, everything makes sense, and then you read the comic The Search where you learn that
Zuko is not even the son of the fire lord Ozai. Now it is stated in the comic that Ozai knew this, but it was not stated if Iroh did.
If he did know this, why would he give Zuko advice that he knew would not be relevant?
If he did not know, why did his advice work?