GPSs always overestimate

[dominion]

Also, aren't race distances often based on GPS readings these days?

Some races might be, officially measured courses aren't.

http://www.coursemeasurement.org.uk/

http://www.coursemeasurement.org.uk/gps.htm

[stumpy]

Reporting the same distance repeatedly is not the same as reporting the correct distance.

Exactly - this is the difference between precision and accuracy, something my old physics teacher would bang on to us about constantly!