All GPS watches are inaccurate... The accuracy of them depends as well on the capacity of signal receiving which depends as well on the weather and athmosferic conditions and also if you are in an open area with or without trees and buildings.
Also, if you have curves and turns, the GPS is not able to calculate exactly the curvature or the radius, it depends on the algorithm they use.
The most accurate device is an athletic track and a chrono watch.
Cheers!