Steam Market buy order functionality

Well, the FAQ tells us the following:

How do buy orders choose which listing to buy?

When you place a buy order, the market first looks for all the cheapest items that can fulfill your order. Then the oldest listing (ie. the seller who has waited the longest) is selected and purchased. If the items are listed in multiple currencies, the amounts are first converted into your currency before being selected (ie a 0.03 RUB listing has no priority over a 0.03 USD listing)

That means, the price you list as a buyer, is the amount you are willing to pay max, but you can get it cheaper, if someone is willing to sell at a cheaper price. An item is always sold at exactly the price, that the seller chose. As the buyer, you can get lucky, if someone offers an item below the amount that you set.

So it basically comes down to the order in which the buyers set their offers. When someone offers to sell his item (in your example for $11.41), then the first person that set a buy order with at least this amount will get it. So in your example the guy with a buy order for $12.12 will get the item for $11.41.

Because of that, it doesn't matter how high the highest buy order is, the item sellers dictate the prices and sometimes people just sell really valuable items at low prices, either because it's an error on their side or they just don't know about the real worth.