Because the odds of no woman conceiving on New Year's Eve 1948 was, effectively, zero. As far as fromdownunder's mother conceiving on that date, that was much less likely, but still not all that improbable. If Teapot Happens' account is accurate, on the other hand, the sequence of events was unlikely to happen to anyone.
But if another sperm had impregnated my mother, I wouldn't be posting here, or if other events occured over the past 60 years rather than the ones that did I wouldn't be posting here. The cumulative chain of events and the odds of everything (not just my birth) that led to my post would be trillians to one.
Here is a case of what might be called synchrosity:
One Friday night in the early sixties I went into the city. It was something I had never done before, and I just did it out of impulse. I ran into some friends who were talking about forming a rock band. I said "do you want an organist?" They decided that it would be a good idea, and we formed the band.
Two years later, while playing at a dance, I met my ex-wife, and we eventually married and had three children. Had I not made an impulse trip into town that night two years earlier, none of the events would have ever happened - and these particular three children would not exist.
Of course, something else would have happened between the mid 1960's and 2009, but I would, most likely be an entirely different person, quite possibly still a member of my Church, married to somebody else and with children. And this may have been as equally improbable as what happened, because seeing improbabilities in retrospect is quite easy.
Norm