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.
Well, my take on this is it actually should be a question (do you understand the recipe?). Saying it as a statement wouldn't be used in conversation unless I'm saying it from a place of surprise. Like if I were helping somebody learn to cook and they didn't know what certain terms in the recipe like "simmer" or "blanch" or even the simplest terms like 2Tbsp is two tablespoons but 2tsp is two teaspoons. Then they finally get it and I say "you understand the recipe!" Some of the sentences they give us are weird, lol.