This is because hair on the face is more resistant to treatment than hair on the body. It is also because the face has more hair follicles than the body, which means there is more hair to treat in this zone.
You should also be aware of the affects of hormones in the body, as these can affect the treatment sessions required, or for example the type of hair you are treating: a beard area on a man for example is often more coarse than the hair on a woman's face – so this would mean that the man would need more sessions than the woman as this type of coarse hair is harder to fully remove.