Okay I thought some of this would be obvious (and yes its been discussed before) but the discussion rages on for some reason.
First lets just ignore the fact that trading 47 gold for 48 gold is absurd in an of itself.
The AI is programmed to give specific weight to all you offer and all that it has. This is why when you are ahead by twelve techs but you want to buy Communism from another Civ, that the civ will demand three techs in return or a whole buttload of cash. That's the way the AI trades. "You want what I got? Well fancy that mister human wants something from me. Well fleshboy, you gotta pay through the nose for it because you have lots of things I want and I'm programmed for selfishness."
So in this instance, the AI is not programmed to weight the fact that 47 equals 47 gold, because its a ludicrous trade. It considers that you have more gold than it does and in its mind his gold has more weight than yours and if you offer him gold he's not going to consider it as valuable, but then again the AI is thinking that you are going to trade gold for tech or something like that. The Firaxis programmers have better things to do (like figure out how to airlift settlers/workers/leaders/artillery) than to recode the application to reanalyze trades like this.
Perhaps its the AIs way of saying "Just give me 1 piece of gold as a token, stupid!"