Those are completely different things, Don. Your image is 13th century (see the clothing) which makes it deeply medieval. A whale is a large fish (for them) so if you've never seen one (they haven't) you draw it like a large fish. With scales.
If, however, you are a post-medieval inhabitant of a New World colony, your perspective on realism has become different (renaissance and all that). If you spot a strange creature with
bands, you will draw it with bands.
The first known European depiction of a sunflower is by Dodoens in 1568. Realism, within the limits of the medium.
After the 15th century, correct observation became very important. People would have drawn the creature as it was observed, not as it was supposed to be in their christian mindset. If they spotted an armadillo, they would have drawn it with bands, because those are its most salient feature. Heck, in Dutch they are called "gordeldier", "belt-animal".
"BUT what if the Voynich author was unable to draw well?"
I'm glad you ask, because that's where it gets totally ridiculous (as if it wasn't already). Tucker "identifies" the illustrator as Juan Gerson, a New World artist famous for his religious scenes on church ceilings. This guy was born and raised in the New World, but learned how to paint in the conquerors' style. If there is one guy in history who would never in his right mind have drawn an armadillo with scales (like the Old World pangolin unknown to him) it was Juan Gerson.