I've found my phone's GPS is hit and miss. Sometimes it's spot on and sometimes I've, "arrived at your destination," in the middle of the road far from where I should be.

Phones use the towers and Wi-Fi to get a more accurate fix. In the middle of a more remote location it probably had a much greater margin of error.

The other cause of error would be that each phone would measure a slightly different location. This would widen the radius of uncertainty.