Why are there different normal values for hemoglobin levels and rbc count in males and females?

Women's mean hemoglobin levels are about 12% less than the mean hemoglobin levels of men. This sex-related difference is observed in many species of animals, such as birds and reptiles. It's believed this difference exists in humans between males and females because men have larger bones, which means they would have more blood cells producing bone marrow. Additionally, men's kidneys have a larger diameter than women's kidneys, which would lead to increased red blood cell production.