My guess, from the houses I went into, is their construction. I have to emphasize these are not houses like any first-world person would think of, as soon as y

My guess, from the houses I went into, is their construction. I have to emphasize these are not houses like any first-world person would think of, as soon as you leave the big city there is very little that is modern about China - they're hapharzardly made with found objects, mud that has been packed and dried, thick layers of grasses woven and turned into sheets of ceiling tiles. Everything is handmade, and I have to imagine they are constantly searching for sources of heat loss and patching them. A lot of people also were burning dried animal waste, which was the worst smelling thing I can imagine enduring 24/7. I would be willing to bet if you are really poor you are also getting third rate coal, with lots of fillers that reduce its caloric output of heat (and it is cheaper).