Actually it's the opposite, and the difference between the two is because the river bed has changed over the time naturally and by human engineering. To quote W

Actually it's the opposite, and the difference between the two is because the river bed has changed over the time naturally and by human engineering. To quote Wikipedia: "Serbia holds the opinion that the thalweg of the Danube valley and the centre line of the river represents the international border between the two countries. Croatia disagrees and claims that the international border lies along the boundaries of the cadastral municipalities located along the river—departing from the course at several points—reflecting the course of the Danube which existed in the 19th century before meandering and hydraulic engineering works altered its course."