Enlarged diagram on f68v2-f68v1?

[ololololo]

The general theory

Or the theory of forgery?

to make the book more sellable

We have no information that the book was bought by anyone in the 15th century. 
By the way, one of its owners (Barshius, I think) said that the book was useless to him and only took up space in the library. I assume that this was how she was perceived at the time - she was very interesting, but unreadable and incomprehensible. She was useless to a medieval doctor.

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But the geography of the world would be equally irrelevant.

It is possible that the author perceived this as basic information that one should know in principle, that one should know to understand astrology, etc.