Muito versão curta: porque quantum.
Richard Watson, fora do universo, um dos criadores do Cyan e no universo um membro do Conselho de Restauração de D'ni, tem escrito sobre isso . Note que a carta é escrita de uma perspectiva no universo:
Many of the interpretations of quantum theory say that until a state of matter is observed, it exists in many states simultaneously - it creates a bizarre "probability wave" that contains all of the possible states of that matter. Therefore, as was proposed in Schrodinger's famous cat analogy, bizarre things happen on the quantum level that allow things like Schrodinger's cat to be both alive and dead at the same time, until one ov the states of observed, locking it in a single state, and collapsing the "probability wave."
What the D'ni seem to have concluded (proved?), is that those waves don't actually cease to exist altogether, instead each possibility continues to exist in an alternate quantum reality (read "parallel universe"), until a state is observed in that quantum reality, and the possibilities not observed in that quantum reality continue to exist in still another, and so on ad infinitum. This makes the universe infinitely complex, with every possible quantum combination since the creation of the universe existing in a quantum reality somehere (even the "unstable Ages"). The Books somehow allow observation of (thus the locking of) and travel to those quantum realities.
So, you can make "unobserved" changes (probabilities that haven't been locked down by description in the Book, or by physical observation in the Age itself) without forcing the Book to link to a new quantum reality.
This is why being careful of contradictions is so important. The problem with contradictions is that the Book attemps to link to a quantum reality that matches a contradictory description, and the closest thing it can find is usually fairly unstable.
[...]
"What about the changes [Catherine made to] to Riven? You still haven't answered that."
The changes made to Riven near the end of the Book of Atrus (pg 268 in the hardcover edition), were a collaboration between Anna and Catherine. Anna's main contribution was probably keeping the Book free of contradictions. Catherine's intuitive (but D'ni rule-breaking) style was so bizarre that earlier Atrus had claimed that her Books wouldn't even work - yet they did.
The daggers which mysteriously appeared around the island, and the lava filled fissures were made possible by her odd style - which I cannot explain. And although Catherine and Anna intended for the lava filled fissures as part of their plan to rescue Atrus while still leaving Gehn trapped in his Fifth Age, the Star Filled Fissure was not intentional or anticipated.
To me, it remains the most mysterious object in all the D'ni histories.
TL; DR: Você pode editar coisas que não foram observadas e que não foram mencionadas anteriormente no livro descritivo, sem quebrar o link. Além disso, Catherine e Yeesha fazem coisas estranhas ao escrever e quebram as regras.
Em resposta a uma consulta de fãs, o Watson disse o seguinte:
I was reading BoD the other night and something finally made sense to me.
When you link, you can link to any place at ANY TIME. In BoA, Atrus linked back to a time before they were ever there.
Sorry to add to the confusion, but linking to an earlier time doesn't explain the situation in the Book of Atrus. Atrus actually linked to a completely separate, albeit remarkably similar, Age.
Gehn's removeal of the phrases in the Age 37 Descriptive Book forced the Book to choose another Age which still fit the description. The Ages were very similar, but distinct.
[italics in original, bold added]
Com base nesses escritos, parece-me que a mudança inicial de Gehn para a idade de 37 anos foi percebida como uma mudança em grande escala no clima local. Porque ninguém tinha a tecnologia ou dados para prever com precisão o referido clima, esta foi uma mudança não observada e, portanto, não quebrou o link. Mas simplesmente negar a mudança não foi uma modificação não observada; Gehn estava pedindo por um mundo no qual a mudança climática nunca tivesse ocorrido. Para fazer isso sem quebrar o link, Gehn teria que escrever cuidadosamente em uma segunda mudança climática para cancelar o primeiro.