I wondered this, too, and could only rationalize it by "the flag being in place" before your birth.
According to the Tips & Notes for this Determiner lesson (the lightbulb), it states:
“O” class possessions include primary relationships; relationships that are in place at birth, akua, makua, kupuna, siblings, cousins, also includes spatial relationships of one’s mauli to objects (often described as being underneath, on top or inside these things) like one’s house, car, canoe, chair, clothes or similar.
“A” class possessions include secondary relationships; relationships that one chooses, spouse, children, grandchildren, nieces and nephews, also includes things that you can choose to possess. If a possession can be both “O” or “A”, err on the side of “O”.
This sort of makes sense to me, except when it comes to "friend" (k'ou hoa). I still think it should be "kāu hoa", but it isn't. In the quote I pasted above, the Tips & Notes does state the word "generally," so I guess "hoa" is one of those exceptions.