Gps is not exact. Can be perfect can be off by 200 ft. Uncle sam incorporates errors on purpose as antiguidance precautions
This used to be a big issue, but I don't think it is anymore.
On X has data from the county as mentioned. The device you are looking at has some error, all are different depending on your unit, antenna, number of satellites, etc. If there was no error with a phone and a $30 service, being a surveyor would be a pretty easy job.
In addition to the error with any GPS, the property line could move due to legal reasons. If a neighbor puts a fence up in the wrong place and it stays there long enough uncontested, the property line could move through adverse possession.