Why do the operating and selling carriers sometimes price codeshare flights differently?

The selling carrier has the right to sell a specific number of seats under its own name on the operating carrier’s plane. For example, United may operate a flight from Dulles to Amsterdam on which both United and Lufthansa sell seats. Due to monopoly laws both airlines must price their seats independently. Prices are set and tweaked constantly based on supply and demand. Because the selling and operating carriers price independently, and have access to different numbers of seats on the plane, each will interpret the demand for the remaining seats differently and price accordingly. Also, the operating carrier knows instantly when a seat is sold, whereas there can be a slight delay in the selling carrier getting the updated inventory numbers. About 50-60% of the time prices will be different between the selling and operating carriers, almost always with the operating carrier being the lower.