Pure conjecture, but I would guess it's random or using some non-sequential algorithm.

I'm 10****** from 2004, but was previously a member from the early '90s with 76******. And my Dad was among the original members from way back when you paid for it, and I'm fairly sure his started with a 1 a well.

Now that I think about it though, the card that started 76 was one that I got from a form in HighLife (or was it BusLife - can't remember) where you tore out a paper version of the card, and unlike LH the number on it was the actual one - not a temporary one.

So maybe that would point to them allocating batches at a time for different purposes (at least historically if not now)...