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Variable costing, also known as direct costing, is a way of calculating the cost of goods sold by only including variable costs. Variable costs are those costs that change with the production level or the number of units produced. In variable costing, all variable costs, such as direct materials, direct labor, and variable manufacturing overhead, are included in the cost of goods sold calculation, while fixed costs are excluded. On the other hand, absorption costing, also known as full costing, includes both variable and fixed costs in the cost of goods sold calculation. Fixed costs are those costs that don't change with the production level or the number of units produced. In absorption costing, all fixed manufacturing overhead costs are allocated to each unit produced, and then the total cost of the goods sold is calculated.

Operating income is calculated by reducing the cost of goods sold from sales revenue. Since the cost of goods sold calculation is different in variable costing and absorption costing, the operating income will also differ between the two methods. In variable costing, only variable costs are included in the cost of goods sold, resulting in a lower cost of goods sold and a higher operating income compared to absorption costing. In absorption costing, both variable and fixed costs are included in the cost of goods sold, resulting in a higher cost of goods sold and a lower operating income compared to variable costing.

As mentioned earlier, the main difference between variable costing and absorption costing is the treatment of fixed costs. Since fixed costs are not included in the cost of goods sold calculation for variable costing, the operating income will generally be higher compared to absorption costing, which includes fixed costs as part of the cost of goods sold. The differences in operating income between variable costing and absorption costing can indeed be attributed to the treatment of fixed costs. In absorption costing, fixed manufacturing overhead costs are spread out or "absorbed" into each unit produced, whereas, in variable costing, fixed costs are excluded from the cost of goods sold calculation. As a result, operating income will generally be higher in variable costing (due to lower cost of goods sold) compared to absorption costing (due to higher cost of goods sold). Therefore, we can agree with the statement that differences in operating income between variable costing and absorption costing are due solely to accounting for fixed costs.