I like this statement. It should help get the word out about GPS vs. certification measurement.
Just for nitpicking (I know, everyone's an amateur editor, right?), I recommend slightly different wording for greater clarity- something like this:
GPS devices work by receiving signals from satellites. The quality of different GPS units can vary, but all of them can be affected by conditions such as buildings in urban environments or heavy overhead tree cover that usually interfere with reception of the satellite signals. Such interference will cause GPS readings to be inaccurate.
Race courses are measured along the "SPR" -- the Shortest Possible Route that a runner can possibly run within the boundaries of the defined course path. Most runners don't actually run the SPR, so the distance recorded by their GPS device will usually be longer than the certified length of the course.
USATF certified race courses are measured by a proven accurate method. This method calibrates each measuring device for each measurement against a steel tape. Multiple measurements under a strict protocol then ensure verification of the course distance.