A local race operator (Hartford Marathon Foundation) has a statement on their website explaining that GPS may not be accurate. Part of it is copied below.
1) Courses are measured by the shortest-possible route available to runners on race day. This means
that the measuring bicycle is ridden within a foot of the curb and turns are “straightened out” as much as
possible. This method is used to ensure that no one runs shorter than the stated distance. With other
runners on the course, however, this shortest route can be difficult to run exactly as measured.
2) All certified courses include a 1/10th of 1 percent (one-thousandth percent) "short course prevention
factor." This is a small extra cushion to again make sure no one runs less than the stated distance. So, in
other words, a certified 10 km (10,000 meters) road race is actually measured as 10,010 meters. For a full
marathon, that means 42.1925 extra meters or a little more than 135 feet. This extra cushion is spread out
throughout the course, not simply added to the start and finish, and is present in ALL certified courses.
3) And, lastly, consumer GPS devices are not 100% accurate. We have found they are typically 1-to-2%
off. GPS accuracy is affected by elevation, the number of turns, tree coverage, tall buildings, bridges and
overpasses, and the quality and quantity of satellite reception. In a half marathon, a 1% difference is more
than a tenth of a mile. So, readings of 13.25 miles or more for a 13.10938-mile certified half-marathon
and 26.50 miles or more for a certified 26.21876-mile marathon are not unusual and are to be expected.
I'm not sure I agree with #2, but it could be used to develop a simple statement that could be put on a certificate.