The border between Maryland and Virginia is actually on the shore of the Potomac river on the Virginia side. Since the survey marker cannot be placed exactly on the edge of the water, an edge that also changes with time, the marker is placed off to one side. You can see the distance to the border on this survey marker.

Jones Point used to be the southern tip of the original District of Columbia when it was first created from land that belonged to both Maryland and Virginia. The retrocession process led to the land that was taken from Virginia being being given back to the state.

https://en.wikipedia.org/wiki/District_of_Columbia_retrocession