Workers in different areas do different kinds of work, so they earn different wages.

It's not the same work.

Actually, if you take the time to read about the system, you'll learn that the reason is very simple: different kinds of work are reserved for different kinds of workers, in keeping with the company's comprehensive and World Bank-partnered anti-poverty goals. Tasks that pay less are routed to workers for whom the pay can still make a meaningful impact. For example, OCR tasks that you can do on a cell phone might be sent to an individual working on a cell phone in Mumbai, while tasks requiring Photoshop expertise might be sent to someone in a city in eastern Europe.