Why do civs request deals that they won't accept?

I've noticed a lot of AI civilizations requesting trades with me that they will not accept with a flavor text saying something like "I cannot accept this deal." Sometimes hitting the button for make the trade more equitable will make them accept the deal they offer, but other times they will want me to give them a bunch of things to satisfy them. Is this a known bug, or just a weird quirk due to AI's not being human?