That map for the Mason-Dixon Line is not correct. The original line was at that latitude but it ended at modern day West Virginia. It was the line of demarcation between Pennsylvania, Virginia, Delaware and Maryland. It was used in congressional debate during and after the the Missouri Compromise to refer to the line of division between slave states and free states which lead to an unofficial expansion. Since the 1820s it has been understood to move directly north from it's original endpoint until it hits the Ohio River then to follow the river west to the Mississippi River then to travel along the eastern, northern and western borders of Missouri. It ends on the 36°30' parallel and extends straight west through the Louisiana Purchase. The 36°30' line was applicable in the territories but not among the states. The Mason Dixon was the line of separation among the states.

https://history.howstuffworks.com/historical-events/mason-dixon-line.htm