Economist: In 2015, the average per-person amount paid for goods and services purchased by consumers in Country X was the equivalent of $17,570 in United States dollars, just 30 percent of the corresponding figure of $58,566 for Country Y. Yet in 2015, there was already a substantial middle class in Country X that had discretionary income for middle-class consumer goods such as new vehicles, computers, or major household appliances, while a significant portion of the middle class in Country Y did not have sufficient income to purchase such items.

Which of the following, if true, most helps explain the discrepancy in the relationships described by the economist?