Why is the "Calorie requirement" in urban area less than that of rural areas for determining the poverty line?

Physical work is more in rural areas

While determining the poverty line in India, a minimum level of food requirement, clothing, footwear, fuel and light, educational and medical requirement, etc., are determined for subsistence. The present formula for food requirement while estimating the poverty line is based on the desired calorie requirement. Food items, such as cereals, pulses, vegetable, milk, oil, sugar, etc., together provide these needed calories. The accepted average calorie requirement in India is 2400 calories per person per day in rural areas and 2100 calories per person per day in urban areas. Since people living in rural areas engage themselves in more physical work, calorie requirements in rural areas are considered to be higher than in urban areas.