TLC Measurement - Helium Dilution vs Plethysmography Discrepancy

A 63-year-old former smoker with GOLD stage 4 COPD and bullous disease on prior chest CT undergoes simultaneous helium dilution and body plethysmography to measure total lung capacity (TLC) for consideration of lung volume reduction surgery. The helium dilution yields TLC 7.2 L (105% predicted), while body plethysmography yields TLC 9.8 L (145% predicted). The pattern shown at **B** in the diagram—where plethysmography TLC exceeds helium dilution TLC by 2.6 L—is most consistent with which pathophysiological mechanism?