A 62-year-old man with COPD presents with chronic hypoxemia. Arterial blood gas shows PaO₂ 55 mmHg, PaCO₂ 65 mmHg. Chest X-ray reveals emphysematous changes with areas of reduced perfusion. A 28-year-old woman with acute pneumonia presents with fever, cough, and hypoxemia. Her ABG shows PaO₂ 60 mmHg, PaCO₂ 35 mmHg. Chest X-ray shows consolidation with normal perfusion to affected areas. Which finding best distinguishes the ventilation-perfusion mismatch pattern in COPD from that in pneumonia?