I'm no expert on surveying, but I know that certain areas have stronger gravitational pull and repeatedly survey differently. Where we live in Whatcom County, WA, there are areas that get different results with every survey. It's especially troubling in sub divisions where a few feet can make a big difference.
Whatcom? Isn't that where the creeks flow with gasoline?
Apples and oranges. What is called the gravimetric "sphere" affects compasses as well gravity readings. GPS is immune to all that since the final position depends entirely on measured distances, and is further "projected" onto a standard "geoid", so, all other factors being consistent, repeated measurements at maximum resolution should agree within half a centimeter.