You can visit the GCM site (from where route maps are taken) and read about it in their FAQ:
http://gc.kls2.com/faq.html#map-choiceIn short, they have a set number of pre-drawn maps and use the one that best fits the request.
Rather than quoting, I suggest you read (link provided) the answers to the questions "Why is the world map sometimes used for routes in a single hemisphere?" and "Why not generate appropriately sized maps on the fly?"