No text, but a visual code

[ReneZ]

Antonio, of course there is a difference between educated guesses and speculation, and there is a whole range in between.
Also the follow-up post of R.Sale has a mixture of facts and opinions.

We have a few more facts about the zodiac illustrations.
It starts with Pisces, and this is real. It cannot be the result of moving pages around or incorrect binding. No matter how one re-folds the few sheets, Aries will always follow Pisces. Also, the order of the signs is the correct one, and the pages clearly have not been moved around in this part.

It is also a fact that there are missing pages for the missing signs.
It is possible that the missing signs were on that folio, but we cannot be certain.
It is also a fact that there are plenty of manuscripts with incomplete zodiac cycles, and I am not aware of any cases where a special meaning has been given to this, apart from them having been lost, or the copy being incomplete. However, it could be that there are such cases, and I would be interested to know.

I have my own hypothesis about the doubling (splitting in halves) of Aries and Taurus, but I will not even try to argue that this is more likely or less likely than a deliberate emphasis on these signs.

The MS pages are taller than they are wide, so the size of a circle is limited by the width of the page. In the case of Pisces, there happened to be a more or less square page, so the 'author' could draw a large cirlce.
For the next sign(s) he decided to make the circles smaller to fit on normal pages. After having done Aries and Taurus, he decided to compromise and cramp the larger circles on the smaller pages.
As if he started off ambitiously, but decided to finish it more quickly.
The reduced quality of the later circles matches with this possibility: no more painting, and less attention to equal spacing between the nymphs.

But it remains a hypothesis.