Umm probably.....I think what you mean is: can you explain why people are more likely to bleed more with a physiological third stage rather than having syntocin/syntometrine/ergometrine?
Okay, so the baby is out, the next stage is that the placenta detaches from the womb and heads for the exit. This leaves an open wound inside the womb which will heal gradually but obviously because the cervix is open and the womb is big (it had a baby in it just a few minutes ago), there is a lot of space for the blood to flow out.
With the drug route, the uterus is encouraged to clamp down faster (NB it does clamp down naturally especially if you breastfeed but not as fast as with drugs). So less blood can flow out.