Different countries have different poverty lines because the cost of living varies form country to country.In some countries the income required to live a decent life(above poverty line)is lower than the income required to live a decent life in another country.The price of commodities and the cost incurred for accessing heath and education facilities are different in different countries.
Each country has a socially accepted minimum standard of living that is defined by the costs of basic needs in that country. What constitutes basic needs in one country might be far above a basic need in another, so you tend to see the highest poverty lines in the richest countries. But the process for determining what is included in the basket of goods is unique to each country that sets its own line. There is a world poverty line that adjusted from $1 a day per person to $1.25 per day in 2005. this is a number used by economists when a poor country has no official poverty line established.