GPS satellites transmits their signals in space with a specific precision, but what you get on earth depends upon various factors, including atmospheric conditions, signal blockage, satellite geometry, and receiver design quality.
For an instance:
Mostly, GPS-enabled smartphones are usually accurate to within a 4.9 m (16 ft.) radius under clear sky. But their accuracy degrades near bridges, high-rise buildings, and trees.
The GPS positioning accuracy can be degraded by certain factors. Some of them include:
Satellite signal blockage because of buildings, trees, bridges,
Indoor or basement use
Signals reflected off walls or buildings
Wrongly drawn map
Radio interference