Linking Theory is largely built on the principles of Quantum Physics, so this is going to be wild, a bit hard to follow, and probably more confusing than just saying "the book has it right", but here we go

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First, some background on where all of these Ages come from. Contrary to what Gehn says, the Ages are not created when they are Written. Rather, they link to a pre-existing possibility of an existing world in some parallel universe. There are, for the sake of not having to crunch numbers, a virtually infinite number of these parallel universes, and they multiply at a phenomenal rate each time a particle (or anything larger) needs to perform one of several actions. According to a branch of quantum physics, there is a rule which holds that each of these actions must be manifested, but since only one can be manifested in our universe, the others are manifested in universes that are "spawned" by that paricle's having made a decision. This is the basis for the Great Tree of Possibility. As you branch out from the "trunk" of the tree, you run into various differing worlds that all derive from a common "branch" from the main tree. For instance, you could find a group of "branches" on the tree where the White House was actually painted cherry red, or indigo blue, or was made out of unpainted brick; small variations in the world that often times have little significance. If you tracked farther up the branch toward the root of the tree, you might find more fundamental differences, such as an Earth where the Confederacy won the Civil War, or Germany won WWII, or even where life managed not to spring up at all. In these worlds, you and I may still exist and live in the same places, or our experiences may be slightly different, or we may not have been born at all. It all depends on how far back up the branch you go.
Similarly, when Gehn Wrote the 37th Age, his description (bumbling and contradiction-riddled as it was) drew the link down a branch in the tree that led to Age 37 as it was when Gehn first arrived there. His actions and changes further pushed the Age down its own path on the Tree, seperate from the other branches. Additionally, it is also possible to "nudge" an Age down a branch you want it to follow by writing additional descriptions into the Book, as Gehn did with the Mist Wall and as Atrus did with the soil bacteria. However, by scoring out the lines he had just written when he edited out the Mist Wall, Gehn forced the link to backtrack *up* the branch instead of continue to move *down* it, and the backtracking resulted in the Age coming out on a different branch entirely, where Gehn had never visited the Age, but a branch which was close enough to the original one that the same inhabitants still existed.
I hope that didn't confuse you farther

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