Maps Are Not Perfect
Maps have been used by humans for hundreds of years. As technology has improved, so have the quality and accuracy of maps. Ancient maps were usually drawn by explorers. It was impossible at that time for anyone to leave the Earth and look down at the huge continents below. All they could do was walk around the different land formations, and then do their best to draw what they thought the land probably looked like.
In the modern world, our ability to view and map the Earth is much improved. However, even today it is impossible to draw a flat map that is 100% accurate. This is due to the impossibility of recreating the surface of a round planet on a flat map. The smaller an area that a map represents, the more accurate that map will be.