How can some USB chargers produce larger total amperage than others?
Cost and quality of components. A 2.4A supply should cost more than a 1.2A supply. However, the 2.4A supply might be less expensive or the same cost, if the 1.2A supply was made in fewer numbers, due to economy of scale. Also, a 2.4A supply might cost less because it is made without regard to safety or adherence to standards.