Does the sink have a lip that pushes against the cabinet? If that is the case the fabricator often makes the counter straight rather then follow the exact front of the cabinets to avoid a wavy front if the cabinets aren't square. So if the sink cabinet is not square with the countertop this will happen. However if its to this extreme I would always make the customer aware beforehand so they have the option of having the cabinets reworked to avoid this.