Think of it as the Heisenberg uncertainty principal, the more you try to define something, the less sure of it's position you are. That's what I like to think of for the theory behind writing ages.
I also like to think that that is Gehn's problem—apart from the fact that he doesn't understand the principles, and only copies from existing phrases in existing works (Book of Atrus). I think he tries too hard, and his failed experiments in Riven are a result of this, In this case the link is so badly deformed that it does not even work.
It also seems from both the Book of Atrus and the Stoneship journal, that once the Link has been created, you can alter in slightly to make real changes in the world, as Catherine is able to write the daggers into Riven, Ghen is able to alter the 31st (might have number wrong) understructure, and Atrus is able to create the ship in Stoneship, even if all of these have unintended consequences (The Star Fissure, the Ocean draining and the ship becoming snagged). But only up to a point.
It seems from the book of Atrus, that when you make changes to the age, it warps the link, change it a little and you can get away with it, but as we know, in BOA, Ghen damages the link so far, that even when he undoes the changes that he made, and in theory the Age should have reverted to what it was like when they first linked there, it instead snaps to a similar but different age. One where they had never visited and the people had become savage. This would also explain why Atrus considers Inception destroyed, when in theory, he should have been able to negate the changes his father made. The tampering would have damaged the link so badly that the age he linked to would never have been the same (as an aside, that same scene also illustrates that you can be fairly specific in the writing, since Atrus mentions that he wrote those specific flowers into the age).
I know that you could argue that when making those changes you are making slight adjustments from one world to the next, but that raises a whole lot of paradoxes (are you a different stranger constantly through Riven as Atrus makes constant changes to the descriptive book?), and is basically irrelevant anyway. If a person from within the age, can consciously observe a change, and remember what the age was like before and after, and has no ill effects, and there is a ton of evidence that this is the case (since all of Riven is basically a showcase of this principal), then to all intents and purposes, you ARE actually changing the world.
THOSE TIN CANS ARE NO MATCH FOR ME!!