A 58-year-old male smoker with 40 pack-years presents with progressive dyspnea and chronic productive cough for 3 years. Spirometry shows FEV1 52% predicted and FEV1/FVC ratio of 0.68 post-bronchodilator. The structure marked **A** in the diagram represents a non-significant bronchodilator response (FEV1 improvement <12% post-salbutamol). Which of the following best explains why this finding, combined with the post-bronchodilator FEV1/FVC ratio, is diagnostic of COPD rather than asthma?