Well, as we all know, the Earth is a sphere. Trying to put a 3-dimensional object onto a two-dimensional surface (the map) stretches or distorts certain parts of the map. The purpose of the map, as in what it is attempting to show, like the shape of North America, for instance, changes what is acceptable to be distorted by the map. For instance, Greenland is stretched a lot in conventional 2-D maps because it is close to the North Pole; however, the conventional map conveys fairly well the shape of the continents, so it is acceptable for Greenland to be distorted. On the other hand, if people wanted a map accurately showing Greenland's size and shape, but not that of other countries, perhaps there would be a Greenland-centric map, with distortion of North America and of Europe and Russia in the corners; in that case, Greenland cannot be distorted, because the shape is desired, but the shape of other landmasses can be distorted; they're not the main focus of the map.
