Thee correct answer is - latitudinal differences in scale.
The Mercator projection map is the most popular and the most used map globally. That is somewhat weird as the map has its deficiencies and is not the most accurate one on the market. What makes this map so widely used is actually that it is the most practical.
This map projection depicts the shapes very well, and it is very precise when it comes to the places on the low latitudes, but that drastically changes with the places on the higher latitudes. The higher the latitude the least accurate the projection becomes, as it stretches the territories more and more and giving them proportions much larger than in real life. The reason for this distortion is that the Mercator projection is not well made to cope with the increasing scale as the latitude becomes higher, especially not with the poles themselves as the scale goes to infinity at those places.
