How are/were KF numbers assigned?

[Savage25]

Registered online in 2001, with KF number 800 1*.

Ditto.

[StarG]

I don't think this is necessarily true. Mine starts with 804 and registered in SQ's website in 1999.
Not sure what my parents got as an ex-passage member.

I am reasonably sure that all 800 XXX XXXX members joined KrisFlyer via SQ website.
I am not saying that all who joined via SQ website MUST have an '800-' prefix.