You see that in maps all the time. Sometimes, when mapmakers got beyond areas explored, they... just... kind of... trailed off:



Other times, they'd make straight lines like in your picture. I don't think those are meant to be seen as coastlines. For example, they put Japan (?) somewhere around Arizona. Besides, it looks like the map is more decorative than informational--fully a third of that image is fanciful pictures of Vespucci and personifications of wind and shit. That's not what people sailed by; you should think of it more as a decorative map like this:



Two art history posts in one night. HOLY SHIT the education has paid off.