If google maps is to be believed (If!) then it looks as if the border/boundary ran along a river that has subsequently changed its course, either by nature or by art. There are countless examples of parish boundaries in this country that have wiggled through fields for centuries for the same reason. Only when that “parish boundary” is suddenly promoted to the status of an international frontier does it matter (and is the probably put in the “too difficult” drawer to sort out later unless there’s a very pressing reason to do something).