OAR clarification

Pls could you clarify this –

My question is – for the below example, why is the variable OH cost was not considered? I thought that both fixed and variable OH cost is considered for OAR calculation.

A company has budgeted the following unit costs and revenue of a product:
$
Sales price 50
Variable production cost 18
Fixed production cost 10
In the most recent period, 2,000 units were produced and 1,000 units were sold. Actual sales price, variable production cost per unit and total fixed production costs were all as budgeted. Fixed production costs were over absorbed by $4,000. There was no opening inventory for the period.

7. What would be the reduction in profit for the period if the company had used marginal costing rather than absorption costing? (Answer in $)

The correct answer is $10000.

Reduction in profit if marginal costing had been used is $10000.

Increase in finished goods inventory (2,000 units produced less 1,000 units sold) × fixed production cost per unit = 1,000 × $10 = $10,000