Kapusta: I believe they compared the school aged children/graduates to get the numbers, but perhaps someone can enlighten us. Also, the discussion is mostly about the Yeshivish crowd, so if your experience says otherwise you may hail from a different crowd or it’s simply a “fluke” as statistics aren’t indicative of each individual’s experiences per se.
I think the words DY used you were referring to about different birthrates was “replacement rate”, which he used as shorthand for the whole gender disparity issue, but I’ll let him answer that…
And about the less girls than boys born as a Chesed to allow for a gender gap, it actually makes sense since girls mature earlier and are responsible for Mitzvos sooner because of it. But we’re talking a one year difference, not three or four… So perhaps that year difference between Bar and Bas Mitzvah should be our guide.