I have seen this also. Not sure what triggers the AI the make an offer with something he really don't want to trade away. I actually like seeing this happen once in a while. Anytime the AI contacts me with an offer, I always wonder if this is one of those one time/take it or leave it offers.
I do the same thing to the AI to some extent. Last game I pulled up Catherine and made a tech trade offer. It was slightly in her favor and she responded with the obligatory slap. After that, I would not give her my steam engine tech, even if she returned the same offer. And she did try to make a trade about 10-15 turns later that was "fair", but I was still irked and THAT tech would never be put back on the table.