That is not the meaning of what Bhishma said.
From Satapatha Brahmana:
atma vai putra-namasi
Son is called Atma.
How can Man be same as Son when there are 2 distinct jiva-atmas ?
Now, we have to dig into what the word 'Atma' means.
I can't find reference, but heard in upanyasam, it means:
That which attains what it wants
Now, we can see why Vedas say so - whatever desires a man was unable to fulfill in his life, he can fulfill through his son, like buying own house, or becoming a big business man or musician, if he educates and drives his son to accomplish those goals, he gets the same satisfaction or even better, than if he attained his own goal. We see this many times in real life, whether father really wanted to be a cricketer, but his father didn't have the means, so he gives his son all the opportunity (and also unwanted pressure sometimes).
So, the Pitrus being born again in same lineage is not the meaning of that quote.
Why only male or female children being born, is very complex karmic tangle, that can give rise to opinion based answers, one of which I'm going to give below.
Based on just personal experience, I have noticed that many orthodox brahmana families, where the father & mother performs nitya-anushtana and follow Vedic achara, they happen to beget at least one son, as though given for the sustenance/upliftment of that lineage.
Vedas themselves have a benediction for those who perform Agnihotra:
Pashyati Putram. Pashyati Poutram.
He shall see a son. He shall see a grandson. (Meaning both that he will live long, and his lineage will continue)
Scriptures also say, beget many sons, so that at least one may do Gaya Pinda-Pradhaan.
Then again, for the man who begets a daughter and gives her away at right age to a guna-vaan brahmachari, his lineage get Brahma Loka:
Kanyam Kanaka Sampannam kanakabharanairyutham
Dashtami Vishnave Thubhyam Brahmaloka Jigeeshiya
whereas if you get your son married, all you may get is Naraka in the form of Saas-Bahu ladayi :)
Any case, just because a son is born, doesn't mean Pitru Runa is satisfied, because the duties that son must fulfill are 3:
Gayayam pinda dhaanam (do gaya shraddham once a lifetime)
Pradhyabtham bhoori bhojanath (do yearly shraddham, and monthly
tharpanam - even after having gone to Gaya once)
Jeevitho vakya karnaath (obey them when they're here)