While what VP said is perfectly valid and I have read novels where GPS spoofing was used, there is another possible cause - mapping datums. Typically GPS will use WGS 84.
The GPS shows your WGS position on the built in maps possibly using a different datum.
You find this in UK to with OS mapping based on a different datum.
Historical note:
We once had a target in a particular Middle East country. It was a particular complex in the middle of a city. The posit plotted 16 miles North of the city. No one could tell us which was correct.
Last edited by Pontius Navigator; 14th May 2018 at 20:38.