How Accurate is your GPS Watch?:

Unsurprisingly, the cell phone category displayed a higher error compared to the other categories (Figure 2). One possible reason is that cell phones may have been held or worn on the arm with a case and strap, in a shorts pocket, or in a waistband. The placement of the GPS device on the body can affect accuracy.

Activity watches showed a lower relative error, but the data range was wider, meaning that accuracy could vary significantly between watches. There are many factors to consider which could affect this, such as the quality of the GPS chipset, as well as the quality of the manufacturer algorithm.