What is causing the estimated unit product cost to fluctuate from one quarter to the next? The fixed portion of the manufacturing overhead cost is causing the unit product costs to fluctuate. The unit product cost increases as the level of production decreases because the fixed overhead is spread over fewer units. The fixcd portion of the manufacturing overhead cost is causing the unit product costs to fluctuate. The unit product cost decreases as the level of production decreases because the foxed overhead is spread over fewer units. The variable portion of the manutacturing overhead cost is causing the unit product costs to fluctuate. The unit product cost increases as the lovel of production decreases because the variable overhead is spread over fewer units. The variable portion of the manufacturing overhead cost is causing the unit product costs to fluctuate. The unit product cost docreases as the level of production decreases because the variable overhead is spread over fewer units