Why is the unit product cost different from the cost that would be incurred if another (additional) unit were produced?

The unit product cost would be different from the cost that would be incurred if another (additional) unit were produced because of the presence of fixed cost. The fixed cost is the cost in production that is fixed based on the level of activity but it will change on a per unit basis. Therefore, if additional unit will be produced, the fixed cost per unit would change thus giving you a different unit product cost.