I guess it's because knowing and understanding are two different things, although very similar. I understand all the words in 'Romeo and Juliet' but I don't know them all. As far as I see it, understanding something is knowing what it means, whereas knowing something is storing it in your memory.
I guess that you could say "You know what the recipe means".
Looking at this another way, "I don't know the recipe" basically means "I can't recall the recipe" whereas "I don't understand the recipe" means "the recipe doesn't make sense to me".
That's my take on it anyway.