Of course, they use more accurate measurement devices than GPS (none of which I have seen ever guaranteed accuracy within less than 5% tolerance) and measure the shortest route possible when measuring courses if they do it right. That means you should expect to run more than 10 miles for a “10 mile” race and a reading within 1.9% of the stated difference essentially should tell us it’s as accurate as can be determined with the equipment and method used.
BTW Jerry: I’m not going to see that GPS at Al’s Run, am I? If I do, we’re going to have a little talk after the race. 😉