An interesting observation on Quire 13

[MarcoP]

There is one serious consideration which seems to have been forgotten here.

If the scribe indeed went for the bifolio model, with the bifolios not to be bound on the folio-basis, then why does the text not follow this model and is put down as if the stuff needs to be bound on the folio basis?

There is a good example of f101, where the text flows across two adjacent folios of a tri-folio, both in the recto and verso sides.

Hello Anton, Minta Collins discusses a herbal which apparently was for a long time kept as a collection of unbound bifolios:

"Two further ninth-century Italian manuscripts were probably taken to the Abbey of St. Gall almost immediately after they were copied since they are compilations of medical texts which may be among those listed in the the ninth-century catalogue entries of the library: 'Libri medicinalis artis volumina II e I parvus; Item libri III medicinalis artis in quaternionibus' (i.e. unbound). 
St.Gall Stiftsbibliothek Cod.217 certainly fits the latter description. .... [Bischoff] deduced from the vertical fold down the centre of the manuscript that it had been carried unbound, almost like a roll, with the outer folio (page 252) serving as a cover."

In this case, the manuscript was folded twice: on the central fold that was later bound and on a second fold running between the two columns of the text. The second fold is still visible on You are not allowed to view links. Register or Login to view..
This example seems to me a hint to the fact that even unbound manuscripts were written without crossing the central fold. Voynich f101 is an exception to this rule, but the fact that in the other bifolios text does not run through the central fold seems consistent with the common practice for manuscripts "in quaternionibus".