If it's a medicinal why the lack of accuracy?

[Jorge_Stolfi]

This is a question I posed online.  Are flowers really important to a medicinal document?

Sometimes the flowers are indeed the part of the plant with supposedly medicinal properties.  Chamomile is an example.  You are not allowed to view links. Register or Login to view. should have more examples. 

But otherwise flowers are an essential detail for identifying a plant, even if the drug may be some other part.  There are many, many examples of unrelated plants with similar leaves.  Like oak and poison oak...

Maybe the snake was the reminder of what the plant was for.

Indeed that seems to have been a common artistic convention for plants that allegedly were good for snake bites.  

And you should check Marco Ponzi's transcription of the You are not allowed to view links. Register or Login to view.  (I particularly like the Herba Folio (#48) whose juice can make iron so hard that it will cut any other iron.  And it is proven.  Unfortunately it seems that we lost the page describing Herba Foobaris, whose juice makes iron armor so hard that it will resist being cut by any other iron.)

That book (or book genre -- was there more than one?) was popular around that time.  I guess we could call it "Botanical Fiction" -- like Science Fiction, Historical Fiction, or Geographic Fiction (a genre quite popular until 1800 or so), but with plants instead of planets, kings, or unknown islands...

could it be the artist being the author was not so good with depictions.  ... I conclude that the artist was an amateur.

That would be a very generous assessment of his ability...
 
All the best, --stolfi