Why is standard hemoglobin taken as 15 in post partum females?

For calculating corrected reticulocyte count why is standard hemoglobin taken as 15 in post partum females? Is 15 a standard level in this state?

The standard reference range of hemoglobin concentration in females is 12-15 g/dl. With these calculations, normal Hb can be taken as 15g/dl and normal PCV as 45%.

Thank you.