Hi Ko-Yung Tung. The short answer is inflation. The $1.00 international poverty line was set in 1990, based on the costs of basic food, clothing, and shelter in the poorest countries in 1985. The poverty line was reset to $1.25 based on 2005 prices and reset at $1.90 in 2015 based on 2011 prices. The real value of $1.90 in today's prices is the same as $1.25 was in 2005. Check out these FAQs for more on this. And, for a more in-depth explanation, read this blog post: The international poverty line has just been raised to $1.90 a day, but global poverty is basically unchanged. How is that even possible?