Ret-Con, Rationalizing, And Ockham's Razor
Writing wisely
You have something in your story, and you want to justify it.
Remember Ockham’s Razor: the simplest explanation that covers the facts is best.
In fact, Ockham’s Razor is of more use here than in more scientific matters. In fiction, what matters is plausibility, not possibility.
In science, a law can reign, unquestioned, for millennia until new data proves it doesn’t cover the new case, or until someone devises a new and simpler explanation.
In fiction, it’s not that easy. Yes, you can have the plot turn on a logical twist. But it has to be a new and simpler explanation, or else a clearer set of data that already was hinted at.
This is particularly important for backstory. Backstory is inherently told and not shown. You can not reveal all the ups and downs of what happened that gave it dramatic impact as it unfolded. As backstory, its dramatic impact lies in the single line that turns everything upside down with what it reveals.
Suppose you have a tale in which the obvious suspect has a clear alibi. But, at the end, the writer violates the tenth of Monsignor Knox’s famous rules about detective fiction:
Twin brothers, and doubles generally, must not appear unless we have been duly prepared for them.
Now, if you have clues about what a cantankerous old coot the murder victim was -- and how he alienated all his children except one son, whom most people think was angling for his money -- and how the coot wrote to all his other children, and everyone remembers how he tried to play his heirs against each other -- you have clues.
Mysteries, of course, are engineered toward careful arrangements of clues. Thus they have more leeway -- though as that rule observed, there are bounds.
At the opposite extreme, you have retcons.1 Whether performed by the writer himself, or perhaps more extravagantly, by the fans. Whether to escape a series being painted into a corner, or just trying to change things.
These can be the most tortured contrivances of logic you ever did see. We shall glide gracefully over the truly contemptible ones -- those that purposefully omit evidence and claim unreliable narrators2 without evidence -- and just look at their violations of Ockham’s Razor.
Let us look at how Luke was hidden with his uncle and aunt on Tatooine. They are dirt-poor farmers wringing a living out of a harsh environment. Furthermore, they both knew Luke’s father well enough to compare Luke to him, and he calls them his aunt and uncle.
Ockham’s Razor says that this is where the family originated. Luke’s father grew up here, and they knew him there. At that, the neighbors knew of the family, so that they knew Luke was the nephew.
So the question arises, of why hiding Luke from Darth Vader is done by putting him with his own family where the family lives.
If you assert that’s only a problem with the prequel trilogy, and offer as an alternative explanation that they moved there --
First of all, the Star Wars universe does not appear to have a high degree of intersystem mobility, so it’s not very normal conduct, but that’s possible to pass off as background. (Though if they moved, their old neighbors would remember well enough to talk; they would have to lie about destinations, and depending on what ships go where, a plausible lie might be easy to debunk.)
At that, the normal thing to do when you move to a new location with a baby you will raise as your own is to tell the neighbors that he’s your son. As Leia was raised by her adoptive parents. It’s possible that they had some objection to doing so, but it would need to be a strong one to override the danger to Luke.
Then, in Luke’s discontent, it might be possible that he does not know that they voluntarily moved to such unpleasant circumstances, and it might be possible that when he complains, he does not bring it up as something to grouse about. (But their neighbors would know they moved in, so ordinary gossip would tell him.)
Still, it might be possible that, when telling him to stay put, his aunt and uncle did not bring up whatever story they told him about why they moved there. Given what a place Tatooine was, it would have to be they had either come from somewhere worse -- to observe he could have it worse -- or fled danger that still lurked.
But when they are desperate to keep him there, and in ignorance, they would be unlikely to avoid using any argument that they had to hand. And he would be unlikely to avoid using any argument that he had to hand in his desperation to escape.
I have heard a fan theory that Luke was put there as a sacrificial lamb, with the intent that Darth Vader would find him and, not knowing about Leia, not look any further. On the other hand, then you would have to explain why the bait had not worked, and both children had grow up without being discovered.
All of these theories are possible, but they are not plausible. Furthermore, they require long explanations, which are the death of drama (which requires you to feel) and comedy (which requires the point to be quick).
They might, wrest loose from the Star Wars universe, result in a dramatic tale, which would allow you to festoon them with details -- perhaps their neighbors blamed them for Imperial attention, and lied and claimed they had killed them for their crimes, thus muddying the trail -- perhaps emigration would reveal a health problem that rendered one of them sterile, so that Luke had to be their nephew -- but it would form an entire story, and not a short one.
Nothing you could throw into a letter, or a line of dialog.
This is why you can paint yourself into a corner in an unlimited series so easily.3 It is easy to think up explanations of why things are different this time, but not so easy to think up simple and elegant explanations of why.