How the heck do airlinesdo it?

I am often baffled by how connections in Europe are priced. When searching for an exotic trip abroad, you may find a flight Munich to Seoul v. Amsterdam, for example, for a reasonable cost. Being the canny shopper that you are, you decide that you can just plan to arrange a cheap budget flight to Amsterdam, and then book Amsterdam -> Seoul directly. And then you discover that it's 300 Euros more expensive to fly direct from Amsterdam; even though you'd be on the same flight as if you flew Munich via Amsterdam.I don't get it.