The two most important determinants of this equation are Hb and SaO2, because PaO2 has a minor coefficient and affects the equation only when referring to high depths (where the high atmospheric pressure results to a coefficient much larger than 0.0031).
Given that SaO2 is high (by the question stem), the only variable that may affect the equation is Hb.
Hb levels fall in blood loss and hemoglobinopathy. But in hemoglobinopathy, SaO2 would also be affected (contrary to what the question stem gives), because mutated hemoglobins don't have the same affinity for O2 as HbA does. Furthermore, one wouldn't expect to diagnose a hemoglobinopathy in an adult for the first time.
So, the most probable cause of low Hb levels, that in turn lead to low O2 content, is anemia indeed.