A reduced FVC with a normal FEV1/FVC ratio indicates a restrictive lung pattern, where lung volumes are decreased but airflow obstruction is not present. Pulmonary fibrosis causes scarring and stiffening of lung tissue, leading to decreased lung compliance and reduced lung volumes, manifesting as a low FVC. Since airflow is not obstructed, the FEV1/FVC ratio remains normal or may even be increased. This pattern distinguishes it from obstructive diseases like asthma or COPD, where the ratio is decreased.

Reference: Pulmonary Physiology, Michael G. Levitzky, 8th Edition.