You clearly don’t know about GPS Drift. This is normal and not a fault in the Coxmate product.
Here’s some detail about it from the Garmin website.
Movement or distance gained while not moving
GPS drift is caused by many external factors and consumer grade GPS devices cannot account for these. GPS drift causes the device to appear to be moving on its own. It is most obvious when looking at a track or when zoomed in all the way while at a standstill.
GPS Satellites send their signal through the atmosphere down to earth. The atmosphere distorts this signal, and other environmental factors (such as trees, hills, mountains, buildings, cars, reflective surfaces and more) can further degrade the signal.
In the past, a satellite signal weakened by the environment caused a loss of position. High sensitivity chips were created so that it was no longer a question of whether you had a position lock, but how accurate that position lock would be.
Now, only the weakest signal prevents the device from locating your general position, but as a result, the decrease in accuracy that is introduced by the environment causes GPS drift.
In other words, the device is more sensitive than the environment allows it to be accurate.