The accuracy of the GPS signal can vary due to multiple factors:
- How well the asset can see the sky
- The position of the satellites in the portion of the sky that the device can see
- Local weather patterns
- Localised interference
- Building positioning
For example; a fix on an open road with an excellent view of the sky will be more accurate as opposed to a fix in a built up area. In addition, an initial fix just after the device initalises can be less accurate as it locates further satellites to improve it's accuracy
This behaviour is consistent with non-military grade gps devices on the market.