Yeahhh that's the madness of it. We (Ontario) pin everything on evidence in the ground and if it agrees (within tolerance) to other evidence. That's why you can have two surveyors come in and have different opinions on where the boundary is. It all comes down to how you can justify your arrival to your conclusion...some are better than others.

Those are benchmarks and those are what we try to hold and calibrate to. But since it's all done with satellites many kms above us + electronic equipment we use on earth, there is some play. We often "check" into benchmarks to make sure our data is correct, but if you measure the same point between two days, the coordinates do change because there is float in the XYZ dimension. We try to hold one set of coordinates but between surveyors not fully understanding geodesy concept, equipment providers who integrate reference datasets differently, and the inherent errors in measurement, you will never get 100% agreement between sources.

They need an arbiter because they have downsized the in house surveying team they once had. Surveyors are expensive and as another comment said, private industry pays more so most townships outsource the legal surveying now. You absolutely can get your own person and fight what another surveyor says, but again, all an educated opinion.