flat-top / hollow roots

[Bluetoes101]

Could the plants with the discontinuous root / stem transition and the ones with the nice transition simply be a stylistic difference, having been drawn by separate artists?

Personally I think this is most likely. The differences line up with possible scribes fairly well too. 
We can theorise scribal variation and plant drawing variation go hand in hand based on this. As in, each scribe had certain identifiable tropes with writing and drawing. 
Or we could say each scribe had their own drawer (or other ideas), but I think its much harder to say 1 person was involved and this is all coincidence.