Why is a normal SpO2 98%?

Question:  Why is a normal oxygen saturation 98% and not 100%?

Answer:  After the diffusion of oxygen from the alveoli to the capillary occurs, this oxygenated blood moves to the pulmonary vein to the left atrium.  This blood contains a PaO2 of 104, on average.  This blood constitutes 98% of cardiac output.  Another 2% of the cardiac output comes from the bronchial veins, and this blood has a PaO2 of 40.  This unoxygenated blood is shunted into the pulmonary vein, and mixes with arterial blood.  It is because of this natural shunt that a normal saturation is 98% and not 100%.