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

[Rafal]

I believe that authors of medieval herbals never cared for accuracy of the pictures.

Let's face the truth, in 99% of cases they were some nerdy guys who almost never left the scriptorium   
They didn't walk the meadows and check how these plants really look like. 
They were copying them from another existing herbals, often adding new mistakes to already existing ones without any reality check.

And it was not a problem.

An illustrated herbal was quite expensive and was something much more prestigious than practical. 
It wasn't used to collect herbs. It was supposed to lie on your shelf and make impression on your patients if you were a doctor. Or it was made for rich people just for fun, often going together with bestiary containing dragons and unicorns.

Herbs on meadows weren't collected by educated men carrying herbals. They were collected by village women who didn't need herbals because they had their traditional lore.

It only changed later, in the time of Renaissance. Then plants in the books became more realistic just like any pictures became more realistic then.