A 58-year-old man with decompensated heart failure presents with progressive dyspnea. An upright PA chest X-ray shows blunting of the right costophrenic angle. On subsequent axial CT chest performed with the patient supine, a small fluid collection is noted in the posterior aspect of the right hemithorax at the junction of the diaphragm and lateral chest wall. This location corresponds to the structure marked **C** in the diagram. Which of the following best explains why this region is the EARLIEST site to show radiographic evidence of pleural effusion in upright patients?