This is something I only noticed today. I have tried to find an answer to this question, but apparently no-one on the internet has yet done research on this, or at least Google didn't find it.
So far I have always assumed that, even if the RATE changes, every single gem is still worth the same at any given time. Meaning that if (for example) 1 Silver can buy you 1 Gem, then 20 Silver would (at that very moment) buy you 20 gems.
However, I noticed that this is not true, and even did the math just now. Right now, to buy 1 Gem I would have to trade 1.39 Silver. That would mean that to buy 5 Gems, I would have to trade 6.95 Silver. But I don't. 6.95 Silver buys me 7 Gems instead (meaning suddenly each gem costs less than 1 silver). This means that there is some kind of bulk discount, completely unrelated to the fluctuating rate.
I have tried this several times, and even though the exact number (obviously) fluctuates, one thing always holds true: The larger the amount of gold traded at once, the lower the price for each individual gem. This means that making several small currency exchanges is a disadvantage, the system rewards trading in large amounts of gold.
So this would be the pretty important question: At which point does the discount stop? I.e., at which amount of gold traded at once does the cost for each gem stop decreasing? This would be good to know, as any transaction smaller than that means you are wasting gold. Has anyone ever encountered this threshold, or knows a website where this was already discovered? Or does anyone here have large amounts of gold available and could check (you don't have to actually trade it - just enter the amount of gold and look how much the exchange would give you for it)?. I will start a list like that myself, but since I have almost no gold right now, it will take some time before I get any useful data.