GPS Clock Rates

The Global Positioning System (GPS) consists of a network of 24 satellites in ~12-hour orbits, about 20,000 km above Earth’s surface. Each satellite carries an atomic clock on board. The clocks must all be synchronized with each other and also to the U.S. Naval Observatory Master Clock. A GPS receiver with an accurate terrestrial clock can then measure the signal propagation time and hence the distance from a few satellites and determine its position.

The satellite clocks are apparently built with a 38 microseconds per day lag relative to the master clock, so that when they are in orbit, they run at the same rate as the master clock. What puzzles me is that the clocks are apparently still re-synchronized every day, otherwise errors build up. Is there a discrepancy in relativity theory, or are the clocks just not accurate enough?

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