Why are the hematocrit and blood concentration levels lower in women than in men?

Testosterone. Testosterone causes some proliferation of the red cell precursors at puberty. I suspect this is the result of natural selection as early humans were hunter-gatherers. High hemoglobin improves aerobic fitness (ask a sports cheater), and allows faster running. Survival requires you run faster than the next guy when you're both being chased by a bear.