LOL. Well, I am not sure I have a satisfactory answer on this point. This is a question that is debated among commentators and more often than not simply glossed over. I do not usually have much regard for commentators but
Gill's Exposition of the Entire Bible does have something interesting to say on this that is worth considering. He says,
"A daughter, whether born alive or dead, if she goes with it her full time: then she shall be unclean two weeks; or fourteen days running; and on the fifteenth day be free or loosed, as the Targum of Jonathan, just as long again as for a man child: as in her separation; on account of her monthly courses; the sense is, that she should be fourteen days, to all intents and purposes, as unclean as when these are upon her: and she shall continue in the blood of her purifying sixty and six days; which being added to the fourteen make eighty days, just as many more as in the case of a male child; the reason of which, as given by some Jewish writers, is, because of the greater flow of humours, and the corruption of the blood through the birth of a female than of a male: but perhaps the truer reason may be, what a learned man (p) suggests, that a male infant circumcised on the eighth day, by the profusion of its own blood, bears part of the purgation; wherefore the mother, for the birth of a female, must suffer twice the time of separation; the separation is finished within two weeks, but the purgation continues sixty six days; a male child satisfies the law together, and at once, by circumcision; but an adult female bears both the purgation and separation every month. According to Hippocrates (q), the purgation of a new mother, after the birth of a female, is forty two days, and after the birth of a male thirty days; so that it should seem there is something in nature which requires a longer time for purifying after the one than after the other, and which may in part be regarded by this law; but it chiefly depends upon the sovereign will of the lawgiver."