This can happen for various reasons. For example, problems may arise in diseases of the blood or respiratory system.
After pneumonia, pulmonary fibrosis often occurs when, due to the disease, “breathing” lung tissue is replaced by connective tissue.
This can be compared to closed windows. You'd love to breathe, but fresh air doesn't come through tightly closed windows.
Another reason is blood diseases.
For example, anemia is common. When there are not enough red blood cells or hemoglobin itself, then there is simply nothing for oxygen to move around the body.
In this case, the saturation also falls below normal.