The different countries use different poverty lines because
(i) The calorie requirement of different human races is different depending on their physical condition and dietary habits. Those races which have greater height and built require higher calories.
(ii) The per capita income in different countries is also different i.e. per capita income is higher in developed countries as compared to developing countries.
(iii) The standard of living of Western countries is higher than that of developing countries.
(iv) The cost of essential items used in calculating poverty line is higher in the developed countries.