Accuracy varies depending on the type of the GPS receiver of your smartphone. In general, you can expect that your position will be within 15m of the real position.
There are circumstances where your smartphone's GPS receiver records location that does not accurately represent your activity. This can mean your device simply lost a connection to GPS satellites and did not record any data; that it recorded GPS points that deviate from your true location or path. Keep in mind that while the GSM signal outage does not affect the GPS signal, the device still needs the 2G/3G/4G signal to transfer the data to the app — meaning our servers cannot translate the data from the GPS to the app with no GSM signal available to transfer these data.
There are many factors that contribute to the accuracy of your GPS data (Reflection of electromagnetic waves , line of sight conditions, multi-path propagation, etc), in fact there is some degree of error inherent in any GPS recording.
Environmental factors such as dense trees, steep hillsides, tall buildings, or even heavy cloud cover can impact or even interrupt the travel of the GPS signal between your device and the satellites.
Ignoring environmental factors, different devices do simply have different qualities of GPS hardware and software - meaning that even if your device is working at its peak performance, there will always be a margin of error in the accuracy of its recording.