The existence of Myst, the existence of Myst II and the existence of Myst IV are not canonically possible. There can be two of them, but not all three (and declaring the trap book Myst II non-canon really wrecks a lot of stuff and doesn't really fix the inconsistencies in I and IV).
However, Uru's explanation of "instancing" takes the cake.
Since each player can exist in their own "instance" of any "age", and the books on their shelf all link to their own "instance", and you can only play with other people in instanced ages by using the Nexus or having them use your bookshelf directly, this means that there's an identical age for every single person.
Which is fine, until you realize that the books link to other places that already exist.
This means that:
http://www.mystjourney.com/img/screenshots/uru-06.jpg
...there are thousands of identical tombs in the universe with identical security, identical damage, and identical body of an identical king from an identical background leading identical civilizations in identical giant underground caves...
http://www.mystjourney.com/img/screenshots/uru-01.jpg
...underneath identical New Mexicos accessible by identical volcanoes in the desert.
It also means that there's thousands of these mindscrews floating throughout the universe:
<img width=400>http://www.mtn-man.org/images/URUWide/wide-ahnonay01.jpg
Sorry, no. "Instancing" is a gameplay convenience, the reality is that the written page will link to the closest version findable.