The earth is not an exact sphere which makes more problems for map makers when in pursuit of the best fit for their maps. There are a series of different slightly distorted spherical shapes (spheroids) that are used for different parts of the world. They have strange names like Airey 1881, Indernational 1924 and WGS 84.
I found out about them when I used to work at the Hydrographic Office. The problems come when you try to fit two maps edge to edge and they have been based on different spheroids!

Robin