My main point with the example is that it seems like it would be unfair to penalize someone who wants to have made and sold magic items for a living (even if they now no longer do so) by taking away their XP -- for one thing, you have to figure out some sort of formula to determine exactly how much XP this would be, and so forth, and this seems too arbitrary to be worthwhile, especially when 1 XP which might bring you up a level is at stake. And having made and used a Scroll of Magic Missile in his childhood isn't going to help him now.
But if it's unfair to penalize the male twin, it should be unfair to penalize the female twin, because she did the exact same thing as he did. Certainly it benefits her by giving her a historical prerequisite, but if she gets penalized, so should he, and it seems inconsistent that if she weren't there, he wouldn't be.
And if it is fair to penalize both of them, would they even have extra gp at all? Why wouldn't it be possible to consider those sales part of what make up their Wealth by Level, rather than anything "extra"? What if, rather than selling the scrolls, they used them -- in other words, they acted like regular wizards, except for scribing together? Would you then say that when you give your wizards their allotment of experience, and they use it, then it follows that they have never created a magic item before?
Why does this apply only to XP and not gp? If it does apply to gp, then would a character who used to own a family estate valued at, say, 100,000 gp, but watched it burn down before his very eyes, start 100,000 gp short just because this was in his backstory?
And if a third triplet came in, well, as Souljourner pointed out, at a certain point they may end up having more experience than the third, if they planned it out right...
So this is not exactly cut and dry. Which is my point, and which is why I think that having the allotted XP and gp as a measure of the character's current state is a simpler and more reasonable idea.
Edit: And just to add in the last subtlety -- certainly it's one thing for the GM to say, "Well, you scribed a bunch of scrolls in your history, so I'm giving you less XP." Then arbitrary is fine, and this is well within the GM's right. But to say, "I'm giving you this much XP and your scribing a bunch of scrolls in your history must be paid from this pool now" is an entirely different matter.