They can be by a small amount. The GPS tracking on a dedicate cycling type unit records bread crumb like points. If you are riding an extremally twisted course, such as mt. bike single track and are in a deeply wooded area, the mileage measurement can be off a bit. Riding in heavily wooded area's can cause the GPS signal to get scattered a bit which can throw off accuracy. Many riders add a speed sensor on a wheel hub, which sends a signal to the GPS unit and which overrides the GPS signal. The sensor provides much better accuracy.