Customer requirements and user requirements: why the discrepancies?

Development teams often speak about the need to satisfy the customer and to meet their requirements. But exactly who is the customer, how do they determine their requirements, and how do these differ from user requirements? If the customer requirements don´t realistically (or objectively) present user requirements, can the developers still build a usable product? And if not, who´s to blame when the product meets the requirements of the customer, but not the user?.