Well, yes, the versions we have heard on recordings use 1806. But apparently when it was first collected, it was 1406. It has been folk-processed over the years. Like Joe says, it's an instant clue that it's a tall tale if you keep the older date. Later singers probably like to hold off on that, lead the listener into thinking it's going to be an epic sea tale, letting them gradually catch on that it's a funny song.