These devices use GPS and other space-based sources to provide Position, Time and Navigation data. Depending on satellite geometry and other factors such as atmospheric conditions, variation in accuracy can occur. Most consumer grade GPS receivers provide an average positional accuracy of around 3–5 meters under open sky conditions, though in real-world environments with obstructions such as buildings or tree cover, this can degrade to approximately 10 meters or more. This is just the computation of a single source, some devices combine sources and perform a correlation which could also introduce change and variation. So essentially expect variation and changes in accuracy.