Spirometry — Severe Restriction in Advanced IPF

A 68-year-old male presents with a 2-year history of progressive dyspnea on exertion and dry cough. Physical examination reveals bilateral basilar fine end-inspiratory crackles and digital clubbing. High-resolution CT chest shows bilateral basal and subpleural honeycombing with reticulation and traction bronchiectasis. Spirometry and lung volumes are shown in the diagram. The pattern marked **A** demonstrates severe restrictive lung disease with FVC <50% predicted and preserved FEV1/FVC ratio. Which of the following best explains why the FEV1/FVC ratio remains preserved (>80%) despite markedly reduced lung volumes in this patient?