Because all RTWs and similar products (like Circle fares etc.) are priced differently from one country to another, based, one assumes, on local economic conditions, supply/demand, etc.

The prices can be strikingly different, particularly with RTWs in premium cabins (business or first class as opposed to economy.) For example, a 4-continent business class Oneworld Explorer purchased and begun in South Africa will cost the equivalent of US$5,472 plus taxes and fees. The same ticket (with a different order of stops, obviously) purchased and begun in the USA will be $10,799; in Egypt $6,320, in Japan $7,669, in Australia $10,903, and in the UK $9962. As you can see, sometimes the difference in price is dramatic enough to justify flying somewhere just to start/end the RTW.

The prices change from time to time, and currency fluctuations can also play a major role. There are a couple of websites where you can see the base prices, or you can keep tabs on this forum where this is a major ongoing discussion topic.