Going by this the device must be using DGPS .
GPS accuracy for public use typically ranges from 3 to 10 meters under optimal conditions, such as an unobstructed view of the sky. In urban environments or areas with obstacles, this accuracy can degrade to 10 to 20 meters. The U.S. government specifies that civilian GPS receivers should achieve 4 meters RMS accuracy, with many devices performing better at around 2.5 meters under ideal conditions. Advanced systems like Differential GPS (DGPS) can enhance accuracy to within 1 to 3 meters.