there are two main avenues of interpreting the headcanon that lowbloods form groups that share an “ancestor” (it wouldn’t be called that, obviously, because lowbloods just explicitly don’t believe in those)
those being a) it’s one of the symptoms of lowbloods spawning in huge numbers and being more community-oriented while highbloods are rare and individualistic
and b) that it’s a result of historical omission of lowblood folklore and history; the process of systematically killing and stunting lowbloods before they can achieve greatness and then reducing those who do to mere footnotes resulting in a ludicrously disproportionate, nay, nigh-inversely proportionate ratio of highblood folklore heroes to lowblood folklore heroes
not to mention, of course, the fact that highbloods’ lower populations mean that broodmates sharing signs is way less likely than it is for lowbloods. when you have a neighbour with the same sign as you, it’s hard to say that you’ve got some sort of destinial similarities binding you together.