it is because of the following reasons:
1) the metabolic rate of the neonate is high, so the oxygen demands fo the body have to be kept up. this is achieved by a high Hb value as well as a really fast heart rate.
2) in a neonate, there are more areas manufacturing red blood cells than the adult. in a neonate, the Red Bone Marrow is located at the ends of all long bones, ie the arm, forearm, thigh, and leg bone in addition to the breastbone and vertebrae and pelvic bones. whereas in an adult, this is located only in the latter three areas. as the baby grows up, the red bone marrow in arm,forearm, thigh, and leg bone gradually starts getting replaced by fat (and called as the YELLOW bone marrow), and the only places left to manufacture blood cells is in the breastbone, vertebrae and pelvic bones.
does that answer ur question?