Do Herbal A and Herbal B correspond to differences in imagery?

[MarcoP]

This would mean that A-images were drawn entirely by a different artist than B-images. It is possible, but then why is the difference only noticeable in this small detail. Notably, a detail that is very easy to add once the drawing is in place. Just a swoop of the pen. 

I agree, Koen. I find it difficult to think of more than an illustrator. But then, I also found it difficult to think of more than one scribe, before Lisa's paper. We totally need a peer-reviewed art history paper about Voynich illustrations. In the meantime, it's great to see such an innovative approach: bravo Koen, Oocephalus and SamG!

About the "swoop of the pen", the argument does not apply to "flat-top" or "grafted" roots. In my opinion, Oocephalus is right and the simple dividing-line is a "variant" of the more elaborate flat-top roots. BTW, flat-top roots appear in the Alchemical herbal and Trinity O.2.48 (both probably originating from Italy). 
I don't know about occurrences of the simpler dividing line in other herbals. Wild Sage in Marciana It. Z.78 (North-Eastern Italy, XV Century) has a puzzling "dividing ring".