The AI waned a good deal, you refused, by amending the trade. AI no longer thought he/she could get a good deal.
Unless I'm confused, I think you misunderstood the OP. The OP is saying that the deal moves in favor of the human player.
I've witnessed this behavior on my most recent game, also. I was vastly in the lead--not sure if that had anything to do with it--but the AI would offer a deal, it would show up as balanced, but if I removed any of the items from my side (what I was giving), the deal would become more favorable for me.
In other words, by giving less, I was able to get more.
If I recall correctly, formerly the AI would offer good deals to the human player that, by altering in some way, the human player would miss out on the deal. The opposite seems to be true now. The AI is offering deals, the game shows the deal as balanced, but the truth is, the AI are trying to get more from the human player--but if the human alters the deal, it becomes an exploit and the human can get more from the AI.
This seems like a bug to me.