The real border is in the treaty. They describe geographical features that have sometimes moved ("the river x"), don't exist anymore ("the big oak tree") or take a lot of time to rediscover ("the field belonging to the widow soandso").
The stone is often only near the border - if the border is a path in a forest, you're not going to plonk it down in the middle of the path.
The gis database for administrative boundaries in Luxembourg[0] gets updated once a week, because the borders are based on cadastral measurements, which have been constantly updated since the 19th century. There's an error in every measurement, the idea is that over time it will average itself out.
[0] https://data.public.lu/fr/datasets/limites-administratives-d...
