A 42-year-old woman with severe anemia (hemoglobin 6.5 g/dL) due to chronic gastrointestinal bleeding presents with dyspnea on exertion. Arterial blood gas shows: pH 7.38, PaCO₂ 35 mmHg, PaO₂ 95 mmHg, SaO₂ 98%, HCO₃⁻ 22 mEq/L. Pulse oximetry reads 98%, but the patient remains symptomatic with fatigue and dyspnea. A co-oximeter measurement reveals an oxygen content of 16 mL O₂/100 mL blood (normal 20 mL/100 mL). Which of the following best explains the discrepancy between normal pulse oximetry and reduced oxygen delivery?