Why was I refunded some gold?

I needed a gem to test something and bought just one, priced at 116 gold in the auction house.

When I went to the completed tab to send it to my stash, I had the gem and 16 gold refunded.

Why did this happen?
The paid price lists 100 gold, even though in the search screen it said 116.

Solution 1:

That happened because when your screen refreshed the lowest price for that gem was 116 gold.

And while you were moving your mouse and clicking on Buyout somebody set another gem(the same one) on the AH for 100 gold. The screen was still saying 116, but at the time of your buyout the lowest price was 100, so the game took 116 gold and then refunded the 16 gold that are the difference between the price you paid and the lowest price at that time.

Hope you understood that.

Solution 2:

For commodities that are only purchased on buyout (gems, crafting materials, blacksmith recipes), the price displayed is just the highest amount you will pay for it. In most cases, you get some amount of refund, this could be because:

• Someone else offered it for lower: As @Novarg pointed out, this is probably what happened in your case.
• The order was split into different lots: If you buy 100 Fallen Tooth for 50 gold each (acc. to UI), but maybe there is someone selling 23 @ 35g, another person selling 50 @41g etc. ... then Blizzard automatically splits your order and buys the lowest priced ones first, and then refunds the difference.

Also, I'm not sure if the purchase price displayed is the actual current price or some sort of moving average (or just a heavily cached version that is updated rarely). It seems to give you refunds way too often to be entirely attributable to reason #1.