The gemara says that forty days before the formation of an embryo, a Heavenly voice proclaims bas ploni l’ploni, the daughter of so-and-so is destined for so-and-so. Why is the gemara incongruent? Instead of saying bas ploni l’ploni it should say either plonis l’ploni or bas ploni to ben ploni?
Both the Rema and Maharsha explain that since usually the boy is older than the girl, she is not yet in existence when he is born. As a result, the proper terminology of bas ploni l’ploni is used, and not plonis l’ploni or bas ploni to ben ploni.
In explaining the phenomenon of why sometimes a wife is older than her husband, R' Shimon Aharon Agassi (1852-1914) writes that at times the wife is older because there are souls that are delayed in descending from heaven.