I heard the males tend to grow faster and show sex first so they have time to produce pollen before the females flower. BUT -- I have no evidence or sources to back up this claim and can not verify if it is true or not - though it seems to make sense.
both male and female preflowers usually show at the fourth or fifth week of vegtative growth. Also, if they're given 24/0 photoperiods, they will not show their preflowers as soon as a plant under 18/6.