A 58-year-old man with a 40 pack-year smoking history presents with progressive dyspnea on exertion. Chest X-ray shows emphysematous changes in the upper lobes. Arterial blood gas on room air shows PaO₂ 65 mmHg, PaCO₂ 48 mmHg, pH 7.32. Pulmonary function tests reveal FEV₁/FVC ratio of 0.52 and markedly reduced DLCO. When he is given supplemental oxygen (FiO₂ 0.40), his PaO₂ improves to 120 mmHg. Which physiologic derangement best explains the disproportionate improvement in PaO₂ with supplemental oxygen despite severe airflow obstruction?